SMTP communication problem, 521 Syntax error - Exchange Server 2003

Posted on 2009-04-23
Last Modified: 2013-11-30
Recently, we did a huge server move which included a change in public IP of our E-mail server, even after registering our reverse DNS, we had issues sending to AT&T.  I posted an E-mail to AT&T postmaster requesting an unblock for this errorcode.

There was a SMTP communication problem with the recipient's email server.  Please contact your system administrator.
            < #5.5.0 smtp;521 Syntax error>

AT&T postmaster has come back to tell me that theyre not blocking our e-mails, which leads me back to the 521 Syntax Error, Ive never seen anything like that in an error code before on the 5.5.0 error, what perhaps could be the culprit in this issue?

Question by:megnin
    LVL 11

    Accepted Solution

    521 usually comes from hosts that dont accept mail from you. In your case it looks like your mails to AT&T are sent to an employee like "" (or similar) but your DNS server gives you back as the mx server. You should use some mx tools like or use nslookup from a dialup notebook to check if you are really getting the correct mx record from your DNS server, maybe you might want to try out another (public) DNS server, your provider should have a few you can always use or try out a public one. I think it is very probable either a problem with the MX record or at & t has a problem with their server.
    LVL 1

    Author Comment


    The MX server that is giving me the error message, is actually my mail server, the MX records match with the public reverse DNS just fine.  So there are no issues on my end of the spectrum, It unfortunately seems getting any information or help from AT&T is next to impossible.  I'm at a bit of a loss if they're telling me they're not blocking me though it appears it, and all my MX records are fine.
    Their relay servers show as

    Now I am able to telnet to them just fine, however of course the problem remains I'm still unable to get through their servers.  A sugggestion that had just been brought up to me is perhaps they're blocking me by Name instead of IP?
    I guess the next step is continue to hound their postmaster until I get a straight answer?
    LVL 1

    Author Comment

    Yep, AT&T was blocking us.  We just had to get ahold of the right person to get it resolved.
    LVL 1

    Author Closing Comment

    Thank you.

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