The landscape of email deliverability underwent a significant transformation starting in early 2024 and continuing into 2025. During this period, major mailbox providers like Gmail and Yahoo transitioned what were once considered “best practices” into firm requirements for senders. These mandates include the implementation of SPF, DKIM, and DMARC authentication protocols, maintaining low spam complaint rates, and offering a one-click unsubscribe option. Following this trend, Microsoft’s Outlook.com began rejecting high-volume email that did not comply with these standards in May 2025. Even smaller platforms are now enforcing similar standards on senders.
In a parallel development, Gmail introduced a “Manage subscriptions” feature, which brings unsubscribe options and list hygiene to the forefront of the user interface. Gmail also provided stricter guidance against the use of misleading “friendly-from” names. The cumulative impact of these changes is a shift towards a domain-first, user-centric reputation model. This new model penalizes platforms with poor tenant governance and “noisy neighbors” whose sending practices negatively affect others. Consequently, platforms that send emails on behalf of their customers—such as web hosting providers, email service providers (ESPs), marketing automation platforms, SaaS applications, and e-commerce businesses—must re-evaluate and adapt their architecture, policies, and operational procedures.
This whitepaper provides a practical blueprint for multi-tenant platforms to navigate these changes. It outlines how to structure DNS and authentication, isolate tenants to prevent reputation damage, enforce list hygiene and one-click unsubscribe functionality, and collect critical signals from tools like Google Postmaster Tools, Microsoft SNDS, and Feedback Loops (FBLs). Furthermore, it details strategies for rate-limiting and gating risky email traffic and for managing incident response when deliverability issues arise. The recommendations are based on current guidance and field reports from industry experts like Al Iverson of Spam Resource, official documentation from Google, Yahoo, and Microsoft, and discussions from operators on platforms like Reddit throughout 2025, reflecting real-world experiences.
The years 2024 and 2025 marked a pivotal shift in email deliverability, with major mailbox providers enforcing stricter standards. Gmail and Yahoo now mandate that all senders implement either SPF or DKIM, maintain a valid PTR record, use TLS for email in transit, and keep their spam complaint rates as reported in Google Postmaster Tools well below 0.3%. They also require standards-compliant email headers. For those who forward email or operate as gateways, the use of Authenticated Received Chain (ARC) is required, and mailing lists should incorporate a List-ID header.
For bulk senders, defined as those sending 5,000 or more messages per day, the requirements are even more stringent. These senders must have both SPF and DKIM in place and publish a DMARC policy. While a policy of p=none is permissible, the From: header must align with either the SPF or DKIM domain. A critical requirement for bulk senders is the provision of an easy, one-click unsubscribe process compliant with RFC 8058, and they must honor unsubscribe requests promptly.
p=none
From:
Further emphasizing the importance of user control, Gmail launched its native “Manage subscriptions” interface on July 8, 2025, for its web, Android, and iOS platforms. This feature allows users to easily view senders, see the frequency of emails, and unsubscribe with a single click, thereby increasing the visibility and expectation of RFC 8058 compliance. In June 2025, Gmail also issued explicit guidance against deceptive “friendly-from” names, warning senders not to include calls to action, extraneous reply information, or emojis in the display name, as such practices can negatively impact deliverability.
Microsoft’s Outlook.com followed suit with its own enforcement measures. As of May 5, 2025, Outlook.com began rejecting non-compliant high-volume messages with a 550 5.7.515 error code if SPF, DKIM, and an aligned DMARC policy (with at least p=none) are not implemented. An update on April 29 confirmed that these messages would be outright rejected rather than just being routed to the junk folder.
550 5.7.515
A central theme in this new landscape is the move towards a domain-first reputation model. Mailbox providers are increasingly prioritizing the sender’s domain reputation (“who”) over the reputation of the sending IP address (“where”). Google’s Postmaster Tools places a strong emphasis on domain reputation, and Yahoo’s sender guidance also highlights the importance of domain and DKIM reputation. Reports from operators and community discussions in 2025 have also pointed to shifts in Yahoo’s filtering that appear to be more heavily weighted toward domain reputation.
While not a direct deliverability control, brand signals and accessibility have also gained prominence. Gmail continues to display a blue checkmark for senders who adopt BIMI with a Verified Mark Certificate (VMC) or a Certified Mark Certificate (CMC). BIMI still necessitates a DMARC policy at an enforcement level of p=quarantine or p=reject, which is stricter than the p=none required for Gmail’s bulk sender rules. The increasing availability of CMCs has lowered the trademark barrier for BIMI adoption, which can be viewed as a valuable trust signal.
p=quarantine
p=reject
Accessibility has also come into focus with the European Accessibility Act, which became effective on June 28, 2025. This regulation encourages digital services to be more accessible. Inaccessible email templates can lead to poor user engagement, which is now a primary signal for reputation.
Finally, anecdotal evidence from the field in 2025 suggests that these changes have had a tangible impact. Operators have reported increased difficulty in achieving inbox placement following the enforcement of Gmail and Yahoo’s new rules and during Microsoft’s rollout. Communities focused on cold emailing have reported significant drops in placement and more aggressive spam filtering. While these reports should be treated with caution, they indicate a consistent pattern of a more challenging deliverability environment.
Platforms that facilitate email sending on behalf of a multitude of customers face a distinct set of deliverability challenges. The “noisy neighbor” effect is a primary concern, where the poor list hygiene or sudden volume surge of one tenant can negatively impact the shared reputation of other tenants, particularly when using shared IP addresses or shared envelope domains. While the shift to a domain-first reputation model helps to mitigate the impact of a shared IP, this is only effective if tenants are using their own domains and have proper alignment configured.
Another significant challenge is the heterogeneous nature of sender behavior across a platform. Different tenants may have varying standards for obtaining consent, inconsistent processes for handling unsubscribes, and a wide range of creative quality in their email content. This diversity can lead to eroded engagement signals, making it difficult to maintain a consistently positive reputation.
Multi-tenant platforms are also prime targets for abuse. The scale of these platforms amplifies the potential for compromised accounts, the use of purchased email lists, risky affiliate “growth hacks,” and a higher likelihood of hitting spam traps. As highlighted by Email Industries’ Blackbox, spam traps often find their way onto lists through poor acquisition practices and can cause significant harm to the deliverability of multi-tenant providers.
Finally, the complexity of email forwarding presents an ongoing challenge. Forwarding and mailing lists can break SPF authentication and create complications for DKIM and DMARC validation. While the implementation of ARC (Authenticated Received Chain) can help to address these issues, its validation is not yet universally consistent across all mailbox providers.
To navigate the current deliverability landscape, multi-tenant platforms must adhere to a baseline of “must-have” practices compliant with the requirements of major mailbox providers (Microsoft, Apple, Google, Yahoo – MAGY).
First and foremost is ensuring proper authentication and alignment for each tenant. Platforms should require every tenant to send from a domain they control, whether it’s an apex domain or a subdomain. Each tenant must have a unique DKIM key, and SPF should be configured where applicable. Crucially, DMARC must be enforced with alignment, meaning the From: header domain must align with the DKIM or SPF domain. For any tenant sending in bulk, a DMARC record with a policy of at least p=none is mandatory. Platforms should also guide their tenants toward adopting stricter enforcement policies like quarantine or reject over time. Tools such as the MailChannels domain check endpoint can help providers ensure their customers’ domains are properly configured for delivery success.
quarantine
reject
The second critical component is the implementation of one-click unsubscribe, as specified in RFC 8058. Platforms must inject both the List-Unsubscribe and List-Unsubscribe-Post headers on all marketing and bulk emails. The List-Unsubscribe header should contain an HTTPS URL that, when a POST request is made to it, processes the opt-out request promptly. This is a firm requirement from Gmail and Yahoo for bulk senders and is now a visible feature in the Gmail user interface.
List-Unsubscribe
List-Unsubscribe-Post
Maintaining low complaint rates is also non-negotiable. Platforms should aim to keep the Gmail spam rate, as reported in Google Postmaster Tools, consistently below 0.1% and ensure it never regularly exceeds 0.3%. With unsubscribe friction now being more visible to users within email clients, providing a seamless one-click process is paramount to keeping complaint rates down.
Proper transport hygiene is another foundational element. This includes using TLS for all SMTP communications, having valid forward and reverse DNS records (FCrDNS), ensuring email headers are formatted according to RFC 5322, adding a List-ID for any list-based traffic, and implementing ARC for any forwarding or gateway services.
List-ID
Finally, robust bounce processing and suppression are essential. Platforms must correctly parse and handle 4xx (temporary) and 5xx (permanent) bounce codes. Unknown users should be suppressed promptly from mailing lists. Microsoft has explicitly stated that effective bounce handling is a key indicator of good sender hygiene.
To effectively manage deliverability in a multi-tenant environment, a well-defined architecture is essential. This architecture should be built around several key principles to ensure compliance and maintain a strong sending reputation.
A foundational element of this architecture is the “bring-your-own-domain” (BYOD) model as the default for all tenants. Each tenant should be onboarded using their own domain or a dedicated subdomain (e.g., news.customer.com). This should be coupled with per-tenant DKIM selectors and DMARC alignment. It is crucial to avoid sending tenant traffic from the platform’s own corporate domain.
news.customer.com
Segmentation of email streams is another critical component. Platforms should separate transactional email from promotional or marketing email. For tenants with high sending volumes, dedicated IPs or sending paths should be considered, but only after a proper warm-up period and when the volume and consistency of sending justify it. For other tenants, carefully managed shared IP pools with per-tenant throttling are appropriate. While reputation is increasingly tied to the domain, IP addresses still play a role in throughput and can be subject to localized blocking events.
A robust policy engine is necessary to proactively manage sending behavior. This engine should be capable of gating, or freezing and throttling, email traffic based on real-time signals. These signals should include the spam rate from Google Postmaster Tools, complaint data and block signals from Yahoo, spikes in bounce rates, spam trap hits, and any unusual deviations in sending volume. This allows for the creation of “reputation budgets” for each tenant on a daily basis.
Platforms should also provide a centralized unsubscribe and preference service. This service would be responsible for injecting the required RFC 8058 headers into outgoing emails, managing the POST endpoints for one-click unsubscribes, and logging these events for auditing purposes. Optionally, this service could offer brand-hosted endpoints for tenants through the use of CNAME records.
A comprehensive feedback loop (FBL) and abuse hub is also vital. Platforms must subscribe to all available FBLs to automatically suppress recipients who complain, applying this suppression globally for the specific tenant. This hub should aggregate signals from Yahoo’s Sender Hub, Microsoft’s Smart Network Data Services (SNDS), which is primarily IP-based, and Google Postmaster Tools, which is domain-based.
Finally, for platforms that operate listservs or email forwarders, it is essential to add ARC (Authenticated Received Chain) and a List-ID to all relevant messages. It is also important to set clear expectations with users that, even with these measures in place, DMARC can still fail in certain edge cases involving forwarding.
Effectively managing email reputation at scale requires a strategic and proactive approach. A key component of this is a well-structured warm-up process for new tenant domains, ensuring a gradual and predictable increase in sending volume. Sudden spikes in volume can trigger risk heuristics within domain reputation models, so consistency is crucial. The domain reputation indicators within Google Postmaster Tools should be monitored closely and treated as the primary metric for success. MailChannels customers should use the sub-account volume limiting feature to ensure a controlled warm-up for otherwise uncontrolled customer traffic.
An engagement-first routing strategy is also essential. This involves prioritizing recipients who have recently engaged with emails and demoting or sunsetting those in stale cohorts. Both Gmail and Yahoo place significant weight on engagement signals, so curating lists for quality over sheer reach is a more effective long-term strategy. MailChannels customers should use the engagement metrics API to retrieve and assess engagement so that lists can be segmented appropriately.
Proactive hygiene for spam traps and other risks is another critical practice. Bulk email lists should be pre-screened using risk scoring or verification services to identify and remove pristine or recycled spam traps and role-based accounts. The insights provided by Email Industries’ Blackbox on how spam traps infiltrate lists and negatively impact ESPs can be particularly instructive in developing these hygiene practices.
Governance over creative content and “friendly-from” names is also necessary. Platforms should implement checks to lint for and prevent deceptive subject lines or manipulative “friendly-from” names, such as the use of emojis, faux “Re:” or “Fwd:” prefixes, or calls to action. Gmail has explicitly identified these practices as problematic for deliverability. For the best protection against adversarial impersonation attacks, use LLMs to assess whether the subject line and sender identity is a good match for message content.
Finally, an automated complaint handling process is a must. Any recipient who registers a complaint via a feedback loop should be automatically and immediately suppressed from future mailings. These individuals should never be re-added to a list without their explicit reconfirmation. The overall platform-wide complaint budget should be managed carefully to ensure that a single tenant’s issues cannot jeopardize the reputation of a shared IP pool. MailChannels customers can rely on automated suppression-list management, which can be calibrated on a per-sub-account basis.
To maintain a healthy sending reputation, continuous monitoring of key performance indicators and adherence to Service Level Objectives (SLOs) are essential. Platforms should establish a weekly rhythm for reviewing several critical data sources.
The primary tool for monitoring is Gmail Postmaster Tools (GPT). This provides invaluable data on domain reputation, spam rates, authentication status, and error codes for each tenant domain and email stream. The main objective should be to keep the spam rate well below 0.1%. Any domain showing a “red” or “orange” reputation status should trigger an immediate pause in sending and a thorough investigation.
Microsoft’s Smart Network Data Services (SNDS) remains a useful tool, although it is primarily IP-centric. It is particularly valuable for platforms that utilize dedicated IP addresses for their larger tenants.
The Yahoo Sender Hub is another important resource for staying informed about best practices and any changes to their sending requirements. It also allows for the monitoring of domain and DKIM key behavior.
The one-click unsubscribe rate is a metric that should be tracked, with the understanding that a higher rate can often be a positive sign. It indicates that the opt-out process is frictionless, which can reduce the likelihood of users marking emails as spam. It is also important to verify that the DKIM signature covers both the List-Unsubscribe and List-Unsubscribe-Post headers to ensure compliance with RFC 8058.
Finally, platforms should monitor their tenants’ visibility within Gmail’s “Manage subscriptions” feature. This involves measuring whether Gmail is surfacing tenant emails in this interface and confirming that any one-click unsubscribe actions are being processed cleanly and efficiently.
The challenges and priorities for multi-tenant deliverability can vary significantly depending on the industry vertical of the platform.
For web hosting providers, compromised content management systems and hacked plugins present a significant long-tail risk. A single bad actor on a shared hosting server can instantly sully the reputation of the entire server, causing significant reputation damage to customers who are on the same server through no fault of their own. Within the compromised customer account, if the attacker uses the customer’s domain as the envelope sending domain during an attack (a common scenario), the customer’s own domain reputation can be significantly harmed. Effective control of email abuse from hosting platforms requires a multi-layered approach incorporating accurate content analysis and rate-limiting based on anomalous patterns such as sudden bursts in volume, high bounce rates, or complaint spikes. Because problematic amounts of email abuse can emerge within seconds, providers must implement near-instantaneous feedback loops – a significant engineering challenge at moderate scale. Or, use MailChannels’ automated abuse detection to constrain abuse from uncontrolled client platforms.
Email Service Providers (ESPs) and marketing automation platforms should make the bring-your-own-domain (BYOD) model mandatory for all bulk sending. They must also enforce RFC 8058 one-click unsubscribe for any campaign identified as marketing. A managed warm-up process, potentially starting with a seed list of recent engagers, should be a standard offering. Engagement patterns must be the basis for assessing list and content quality. It’s altogether too easy for poor engagement results to stem from an otherwise “clean” list. Nonetheless, these platforms should implement a hard-fail policy for any list uploads that exceed predefined spam trap or risk thresholds.
For SaaS products that send emails, it is important to split transactional emails from lifecycle or marketing messages by using distinct subdomains and sending streams. For lifecycle marketing emails, one-click unsubscribe should be enabled, and the content should be kept straightforward and non-deceptive. This is particularly important as Gmail has explicitly called out the misuse of “friendly-from” names. As with the ESPs, SaaS platforms must assess engagement metrics. Is a customer’s new email template causing poor engagement that might indicate a high rate of junk-foldering? SaaS companies can’t afford to wait for the complaint rate to tick up.
E-commerce platforms often deal with high-frequency sending, which can lead to a higher risk of complaints. To manage this, it is important to implement frequency caps and have accelerated sunset policies for subscribers who are not opening emails. It is also critical to connect order events with email sending to ensure that transactional emails remain pristine and are sent from their own dedicated domain and stream.
To achieve compliance with the new deliverability standards, multi-tenant platforms can follow a structured 90-day implementation plan.
During the first 15 days, the focus should be on policy and systems. This involves enforcing a bring-your-own-domain (BYOD) model with DKIM for every tenant. For any sender that scales beyond 5,000 messages per day to major mailbox providers, requiring an aligned DMARC record with at least a p=none policy is essential. During this initial phase, the platform should also deploy a one-click unsubscribe service that injects the necessary RFC 8058 headers and processes the corresponding POST requests. For any forwarding or gateway services, Authenticated Received Chain (ARC) should be enabled, and a List-ID should be added where applicable.
From day 15 to 45, the emphasis should shift to segmentation and gating. All tenants should be mapped to specific email streams, such as transactional or promotional, and assigned to different sending lanes, for example, “good,” “monitor,” or “gated.” During this period, the platform should implement reputation budgets and throttling mechanisms for each tenant and domain. An automated system to pause sending for any tenant whose domain reputation in Google Postmaster Tools turns red or orange, or who experiences a spike in complaints, should also be put in place.
The focus from day 45 to 60 should be on hygiene and observability. This involves integrating list verification and risk scoring services to reduce the incidence of spam traps. A codified process for automatically suppressing complaining recipients should also be implemented. Additionally, every tenant domain should be onboarded to Gmail Postmaster Tools to establish baseline metrics that can be reviewed weekly.
In the final phase, from day 60 to 90, the attention should turn to warm-up procedures and education. Staged domain warm-ups should be conducted for new tenants. The platform should also publish concise guides for its tenants on topics such as DNS setup, obtaining consent, unsubscribe expectations, and the do’s and don’ts of “friendly-from” names. To depoliticize these requirements, it is helpful to link directly to the official sender documentation from the major mailbox providers.
When a tenant’s sending practices negatively impact a shared sending lane, a swift and structured incident response is crucial to mitigate the damage.
The first step is to immediately freeze the tenant’s promotional email stream. Transactional emails may be allowed to continue, but only if their authentication and content are verified to be clean.
Next, a triage process should be initiated. This involves a thorough audit of the tenant’s consent sources, a review of their spam rate and reputation in Google Postmaster Tools, and an analysis of bounce classifications, spam trap signals, and complaint logs.
Once the root cause has been identified, remediation measures must be taken. This may require the tenant to reconfirm their subscriber list or to implement a more aggressive sunsetting policy for inactive users. Any deceptive “friendly-from” names or subject line patterns must be corrected, and if a one-click unsubscribe mechanism was missing, it must be enforced.
After the issues have been addressed, the tenant’s account can be re-warmed. This should be done cautiously, starting with a reduced cohort of highly engaged subscribers. The sending volume can be expanded slowly, but the domain reputation in Google Postmaster Tools must remain in a “green” state before normal sending volumes are restored.
There are several common questions that arise from multi-tenant platforms regarding the new deliverability landscape.
A frequent question is whether a DMARC policy at an enforcement level of quarantine or reject is necessary to comply with Gmail’s bulk sender rules. The answer is no; Gmail requires a minimum DMARC record of p=none with alignment for those sending 5,000 or more messages per day. However, it is important to note that implementing BIMI does require DMARC at an enforcement level, along with a VMC or CMC. Moving to an enforcement policy is recommended to better protect the brand and to unlock the benefits of BIMI.
Another common query is about the ongoing relevance of IP reputation. While domain reputation is now the primary factor, IP reputation still matters. IP segmentation remains a valuable strategy for containing the blast radius of any deliverability issues, managing throughput, and interpreting data from Microsoft’s SNDS. However, the focus of incentives and gating policies should be on the behavior associated with the sending domain.
Finally, platforms often ask if email forwarding issues are now “fixed.” The answer is no. SPF breaks by design when an email is forwarded, and DKIM can sometimes fail in transit. While Authenticated Received Chain (ARC) provides a significant improvement, it is not yet universally adopted. Therefore, platforms should still anticipate and plan for edge-case failures related to forwarding.
For bulk senders, a minimum DMARC policy of p=none is required, though moving towards enforcement is recommended: _dmarc.customer.com IN TXT "v=DMARC1; p=none; rua=mailto:dmarc-reports@customer.com"
_dmarc.customer.com IN TXT "v=DMARC1; p=none; rua=mailto:dmarc-reports@customer.com"
It is recommended to bookmark the primary documentation from mailbox providers and other key resources for the most up-to-date information:
Gmail: sender guidelines & FAQ, Postmaster Tools setup, unsubscribe documentation, and the Manage Subscriptions announcement.
Yahoo: Sender Hub for requirements and best practices.
Microsoft (Outlook.com): high-volume sender requirements and details on the May 5, 2025 enforcement.
RFCs: RFC 2369 (List-Unsubscribe) and RFC 8058 (One-Click).
Spam Resource: for ongoing analysis of the 2025 MAGY guide, friendly-from updates, forwarding challenges, and the Gmail subscription UI.
Community discussions throughout 2025 have provided anecdotal but useful insights into the impact of the new deliverability standards. Many have reported noticeable dips in deliverability after the Gmail and Yahoo rules shifted from guidance to strict enforcement. Cold emailers, in particular, have seen sharp declines in inbox placement. There are also theories circulating about Yahoo’s move to a more domain-weighted filtering model. A common theme in these discussions is an increased reliance on one-click unsubscribe and segmentation to stabilize performance. While these reports from platforms like Reddit should be treated as anecdotal, they serve as a valuable “canary in the coal mine.”
For multi-tenant senders, the new landscape in 2025 is one that rewards good governance and engineering discipline. Success hinges on making aligned identity, through a bring-your-own-domain model with DKIM and DMARC, a non-negotiable requirement. Treating RFC 8058 one-click unsubscribe as a baseline expectation is also crucial, especially given its direct integration into the Gmail user experience. A significant investment in per-tenant reputation controls—such as budgets, throttles, and warm-ups—along with robust observability through tools like Google Postmaster Tools’ domain-level signals, is essential. Finally, codifying abuse response and list hygiene as first-class subsystems will be a key differentiator. By embracing these principles, multi-tenant platforms can turn the new rules into a competitive advantage rather than a burden.
This document was prepared by MailChannels. The recommendations provided are a summary of current public guidance and widely observed behaviors as of September 2025. For the latest changes, always consult the primary documentation from the respective providers.
Google. (n.d.). Email sender guidelines. Google Workspace Admin Help. Retrieved from Email sender guidelines – Google Workspace Admin Help
Iverson, A. (2025, April). 2025 MAGY Sender Compliance Guide. Spam Resource. Retrieved from Spam Resource: All Things Deliverability: 2025 MAGY Sender Compliance Guide
Google. (2025, July 8). Gmail launches new “Manage Subscriptions” view. The Keyword. Retrieved from Declutter your inbox with Gmail’s newest feature
Iverson, A. (2025, June). Gmail says: No funny (or smiley) business in friendly from. Spam Resource. Retrieved from https://www.spamresource.com/2025/06/gmail-says-no-funny-or-smiley-business.html
Microsoft. (2025, May 5). Strengthening Email Ecosystem: Outlook’s New Requirements for High‐Volume Senders. Microsoft Community Hub. Retrieved from Strengthening Email Ecosystem: Outlook’s New Requirements for High‐Volume Senders | Microsoft Community Hub
Google. (n.d.). Set up BIMI. Google Workspace Admin Help. Retrieved from Set up BIMI – Google Workspace Admin Help
TechRadar. (2025, June 28). Emails are getting a fresh look thanks to the European Accessibility Act. Retrieved from Emails are getting a fresh look thanks to the European Accessibility Act
Reddit. (2025). Anyone else noticing deliverability dips after Gmail &… r/Emailmarketing. Retrieved from Anyone else noticing deliverability dips after Gmail & Yahoo’s new sender requirements? What are you tweaking?
Blackbox. (n.d.). Spam Traps. Ouch!. Retrieved from Spam Traps. Ouch!
Iverson, A. (2025, September). Email forwarding in 2025: Ugh!. Spam Resource. Retrieved from https://www.spamresource.com/2025/09/email-forwarding-in-2025-ugh.html
IETF. (2017). RFC 8058 – Signaling One-Click Functionality for List Email. Datatracker. Retrieved from RFC 8058: Signaling One-Click Functionality for List Email Headers
Google. (n.d.). Set up Postmaster Tools. Google Workspace Admin Help. Retrieved from Set up Postmaster Tools – Google Workspace Admin Help
Yahoo. (n.d.). FAQs. Sender Hub. Retrieved from Yahoo Sender Hub
Mailgun. (n.d.). What is RFC 8058? How Does it Enable One-click…. Retrieved from What is RFC 8058? How Does it Enable One-click Unsubscribe? – Transactional Email API Service For Developers | Mailgun
IETF. (1998). RFC 2369 – The Use of URLs as Meta-Syntax for Core Mail List Commands and their Transport in Message Headers. Retrieved from https://www.ietf.org/rfc/rfc2369.txt