Why did I get “521 5.2.1 : (CON:B1) http://postmaster.info.aol.com/errors/554conb1.html?”

Posted: Monday, September 24, 2012
Posted byksimpson.
Error code


In the first of a series of blog posts where we attempt to demystify the most common SMTP error codes, today we discuss AOL’s 521 error code:

521 5.2.1 : (CON:B1) http://postmaster.info.aol.com/errors/554conb1.html"

What does it mean?

If you received this error message in your inbox, it means that you tried to send someone at AOL an email message, and AOL’s mail server rejected the connection because it’s seen just a bit too much spam lately from your mail server.

What can I do?

If you are seeing this error message, you need to talk to your email administrator. In all likelihood, your mail server is sending spam on behalf of a compromised user account (i.e. an account which has been taken over by spammers). The spam traffic is making its way to AOL, and AOL has added your mail server’s IP address to an internally maintained AOL IP blacklist. Solution: Locate the account that is sending spam, and shut it down.

You could also sign up for MailChannels Cloud, which is a cloud-based SMTP relay service that detects and blocks spam.

What else can I try?

If you can send through a different mail server, that might help to get your email delivered more reliably. For example, if you have a Gmail account, use the Gmail SMTP server to send your mail. Its host name is smtp.gmail.com. You’ll need to enable TLS security, and provide your username and password in order to use Gmail’s SMTP server. More information can be found on the Google Support Site.

Need help with your mail servers?

Contact us if you have any questions about preventing these errors.

10 Responses to “Why did I get “521 5.2.1 : (CON:B1) http://postmaster.info.aol.com/errors/554conb1.html?””

  1. September 28, 2012 at 10:20 pm, Danil89 said:

    The spam traffic is making its way to AOL, and AOL has added your mail server’s IP address to an internally maintained AOL IP blacklist.

    • March 19, 2013 at 12:52 pm, noscamspam said:


  2. April 22, 2013 at 9:16 am, aszart said:

    Any chance someone can explain how you get off their blacklist?  My account was compromised about a year ago and was sending spam, I’ve since fixed it and greenlit everywhere EXCEPT on AOL?

    • April 22, 2013 at 10:02 am, ksimpson said:

      @aszart To my knowledge, this error is happening because of the bad reputation of your mail server’s IP address. It’s not tied to your user account, because this error message is generated at connect-time, before AOL knows your email address.
      If you’re getting this error, your mail service provider (whomever sends your email) needs to clean up their outbound IP reputation – probably by tightening up their outbound spam filtering.

      • April 22, 2013 at 10:12 am, aszart said:

        @[email protected] but I actually run my own dedicated server and fixed IP and diagnosed/fixed the spamming issue last year.  My server now has a good reputation and no spam sending from it so not sure how to fix it.

        • April 22, 2013 at 10:17 am, ksimpson said:

          @aszart If AOL is continuing to list you, then chances are there is something your system is sending to AOL that is continuing to trigger their limiting mechanism. Even if it’s not “spam” per se, if it’s something that even one AOL user might be complaining about, that could be the cause of all the trouble.
          Have you signed up for feedback loop reports from AOL? If not, visit http://postmaster.aol.com/Postmaster.FeedbackLoop.php and create a feedback loop account. Whenever an AOL user complains about a message from your system, you’ll get an email from AOL with a copy of the message the user complained about. This is really the best way to deal with AOL errors, since their anti-spam system is predominantly driven by feedback from their own users.

    • May 16, 2013 at 2:36 pm, ksimpson said:

      @aszart How long has it been since the compromise was resolved? This error from AOL is not related to your email address, but to the IP address of your mail server. If there are other accounts from your mail server that are sending spam, AOL may block the IP because of that traffic and not because of your account alone.

  3. May 16, 2013 at 12:12 pm, booshir said:

    This is one of the worst policies to be in place. It triggers off of the dumbest things. I have been getting this issue because I sent three emails in response to 3 different people on AOL this morning when I got up, and now I can’t send anything to anyone.
    This is all work related so I’m moving EVERYONE off of their pointless AOL accounts and over to gmail.

    • May 16, 2013 at 2:36 pm, ksimpson said:

      @booshir  The AOL issue seems to have flared up again recently in response to some changes they made internally. We are seeing a huge increase in traffic to this blog post as a result. Good luck!

  4. May 16, 2013 at 7:31 pm, admin18 said:

    It is solely because of the complete incompetence and ignorance of the aol employees. No other provider that I know of has such an idiotic collection of policies.

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