AOL started bouncing emails from my mail server with the following error:

521 5.2.1 :  AOL will not accept delivery of this message.
554 5.5.0 Remote protocol error

On the troubleshooting page AOL asks for specific error code but all I get is a generic message. The emails are sent from Outlook, without any attachments. I don't send out any mass email/newsletters, only personal email.

What can cause this and how should I proceed? Without the specific code I can't proceed with AOL. Have anyone had this problem?

  • Your mail server correctly set up (mailname, reverse ip in dns, stuff like that)? A tcpdump on your mail server, to see the actual smtp dialog?
    – Dan
    Feb 20, 2015 at 12:06
  • Currently this doesn't really constitute a question. Please follow these guidelines to reword your question to something that people may be able to help you with.
    – BE77Y
    Feb 20, 2015 at 12:07
  • @BE77Y thanks for the hint, I added some questions.
    – moorray
    Feb 20, 2015 at 12:31
  • @Dan I tried but the communication is encrypted. I don't want to play around with encryption in the middle of a workday. I guess sendmail will provide the important info in the logs anyways...
    – moorray
    Feb 20, 2015 at 12:33
  • Possibly related: stackoverflow.com/questions/24907191/…
    – Mike B
    Feb 20, 2015 at 15:59

2 Answers 2


You can try submitting the form with no error code selected, and hope for the best. If you're lucky, a human being might read it.

You can also try contacting AOL Postmaster by phone, on +1 703 265-4670 (reportedly AOL postmaster operations are now being handled in India; you've been warned).


Checking around for similar errors, and looking through AOL's Postmaster errors, nothing immediately ties up with what you've reported but doing some cross-referencing (for example from this thread), the linked error codes (521 5.2.1 which you reference in your question) are possibly linked to the 421 CON:B1 error code from the above-linked error code list from AOL, which reads:

21 CON:B1 The IP address has been tempfailed due to a spike in unfavorable e-mail statistics.

It strikes me as odd that they would have ambiguous error code reporting, but since we currently do not have very much else to go on, I suggest that you check your IP reputation using the AOL tool and follow up accordingly.

  • 3
    Thanks for answering. Unfortunately AOL tool reports my IP reputation as Undisclosed and grepping my logs for AOL (aim.com etc) I see only a handful of messages from users who reported the problem.
    – moorray
    Feb 20, 2015 at 15:59
  • @moorray Same boat too. All the sudden it's wonked out, IP rep as Undisclosed. This happened last year too, I'm assuming it's the ambiguous error reporting BE77Y mentions above. You can try using one of these forms postmaster.aol.com/trouble-ticket
    – dhaupin
    May 16, 2016 at 15:00
  • If they are in process of updating your 'reputation' to 'poor' OR they are in process of whitelisting your IP, you will get undisclosed. I can't guarantee this as I do not work there, but have had experience w/ OP problem/question.. and the aol 'postmaster' group in general.
    – B. Shea
    Mar 2, 2017 at 21:59

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