AOL’s misconfigured SMTP servers bouncing emails with 521 error

We’ve fount out there is a problem with AOL SMTP servers that randomly reject emails with this hard error code:
521 5.2.1 "Service unavailable. Please try again later."

In fact this doesn’t happen very often and we may not have noticed something like that even happens if we hadn’t sent newsletters to our customers which counts thousands of emails to domain. In fact, it happened with 9 emails from total 3000 emails sent to but once these 9 people are your customers it may bother you.

From the technical point of view it seems like one of the SMTP servers (suppose aol has dozens of MX servers in cluster behind something like content switch) is not configured properly and responds with 521 error (5xx are hard errors) instead of 421 which causes the email to bounce back to sender immediately. Here you can see the table of SMTP reply codes and as you can see 521 is not defined here which proves 521 error code is not very common. According to RFC1846 521 SMTP error code exists but doesn’t seem to be a real reason why AOL rejects randomly emails.

We’ve considered options how to deal with this awkward behaviour and ended up with reconfiguring our mail server to transform a reply from remote SMTP server from error 521 to 421.

To accomplish this with Postfix you need to run at least Postfix 2.7 or later. We’re running servers on RHEL 5 which comes with Postfix version 2.3.3 and even RHEL 6 doesn’t come with anything than 2.6.6 in official repositories so we were forced to upgrade to 3rd party package of Postfix directly to latest version 2.9.

There was smtp_reply_filter parameter presented in Postfix 2.7 which does exactly what we needed.

  1. create /etc/postfix/reply_filter and insert this line:
    /^521.*/ 421 Service not available, closing transmission channel
  2. add this parameter into file:
    smtp_reply_filter = pcre:/etc/postfix/reply_filter
  3. restart postfix



Posted in Server administration

16 Responses to “AOL’s misconfigured SMTP servers bouncing emails with 521 error”

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  • Evil Kitten says:

    Thank you man! But as we don’t want to filter messages like RLY:SN, only CON:B1, I myself used:

    cat /etc/postfix/reply_filter
    /^521 5\.2\.1 : \(CON:B1\) http:\/\/postmaster\.info\.aol\.com\/errors\/554conb1\.html/ 421 5.2.1 : (CON:B1)

  • aolmarketer says:

    hi can’t send any mail to our AOL customers since 14 jan , cause this error 521 5.2.1 : (HVU:B1)

    ip reputation are neutral

    we updated our system

    and still the same problem

    i tweeted this prblm on but no one answer me

    so if anyone have the same issue or have any idea about this

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  • Linda says:

    I don’t know how anybody can say it doesn’t happen very much. I’ve had it happen 3 times in the past 2 weeks, and those are only the ones I KNOW about, because the senders informed me. I’m looking for another email service – anybody got any suggestions. I already have Yahoo and hate it.

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