What is outbound spam filtering?

Everything you need to know about preserving your mail server's IP reputation.

Service providers and enterprises need to pay attention to the spam exiting their network in the outbound direction. If left unchecked, outbound spam will cause the service provider’s network to be blocked by the rest of the internet — effectively stopping legitimate email traffic for all network customers.

This happens as part of the global effort to stop inbound spam from harming users. Sophisticated reputation systems have been established to track the amount of spam coming from each IP address on the internet. These systems block IP addresses that send too much spam, and rate limit those with questionable sending practices, by blocklisting them.

Outbound spam filtering addresses the challenge of IP blocklisting that occurs when your network is blocked because spammers have managed to infiltrate it and use it to send spam.

How does outbound spam filtering work?

In the world of spam prevention, IP addresses are like social security numbers. The IP address is the only reliable identifying information that email receivers can use to identify the responsible sender of each message. When multiple users send email through one mail server, email receivers on the internet can’t trust anything other than the IP address of the mail server, because spammers can provide fake email addresses or even impersonate legitimate users.

However, an outbound spam filter installed within your own network can be programmed to identify individual users based on their authentication credentials. For instance, whether they entered the correct password to send mail through the server. By tracking individual users, the outbound spam filter can identify spam-like behaviour on a user-by-user basis, and prevent spam from leaking out of the mail server’s IP address. This protects the mail server from becoming blocklisted and legitimate email being blocked.

Outbound spam filtering involves more than just analyzing message content and rejecting the spam. A good outbound spam filter knows how to identify the actual sender of each message, and to record the long-term behavior of each sender, looking for suspicious patterns of behavior. A good outbound spam filter also takes great care not to make mistakes, because mistakes hurt your own users, rather than someone else’s.

How to Choose an Outbound Spam Filtering Solution

Outbound spam filtering solutions are available in a wide range of effectiveness, scalability and flexibility. When evaluating the capabilities of various solutions, here are 10 questions to ask the different vendors:

  1. Will it work with your architecture? The best solutions provide transparent proxying capability to deal with unmanaged services like VPS and dedicated hosting machines, or with subscriber type networks.
  2. Does it scale? Outbound spam volume can dwarf inbound spam volume, with huge spikes in traffic that can disable poorly scaled systems.
  3. Can you manage it centrally? Look for a vendor that provides a centralized management console that brings together configuration and analytics data from all the systems in the cluster.
  4. Does the solution have robust analytics? You want comprehensive sender behavior analysis, plus tools to help administrators review sender behaviour and delivery results.
  5. Can you customize it? You need to be able to write custom policy scripts or rules so that you can rapidly respond to unique attacks from within your infrastructure.
  6. Does the vendor specialize in IP reputation management? They should attend industry conferences such as M3AAWG and maintain relationships with large email receivers and blocklist operators.
  7. Can the solution automatically notify you when an account appears to be compromised?
  8. Can the content analysis system identify potential spam campaigns emerging from your network, in addition to well-known spam campaigns that are already wild on the internet?
  9. Does it let you track sender identity using sophisticated analysis of message headers?
  10. Can you deploy quickly, and will the vendor be there to help you when something goes wrong?


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