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Porch
Asked: 2010-10-03 17:48:37 +0800 CST

550 Requested action not taken: mailbox unavailable

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I setup a small box with Server 2003 64bit to be used as a webserver and email server for a small school. Real simple stuff for a few users. A simple website and a handful of emails. rDNS and spf records setup and pass every test I found including test at dnsstuff.com.

Email sending to almost every email address (google, hotmail, aol, whatever) works. However, with one domain, I get an bounce back with the error.

550 Requested action not taken: mailbox unavailable

It's another school running Exchange judging from some packet sniffing with WireShark. Every email on this domain I have tried sending to gives this error. The email address is valid as I can send to it from my personal, and gmail account without a problem.

Does anyone know of some anti-spam software that gives an 550 error like the above? What else could this be?

Thanks for any suggestions.

Packet capture of the two servers communicating look like this. 

220 <server snip> Microsoft ESMTP MAIL Service, Version: 6.0.3790.3959 ready at  Sat, 2 Oct 2010 12:48:17 -0700 
EHLO <email snip>
250-<server snip> Hello [<ip snip>]
250-TURN
250-SIZE
250-ETRN
250-XXXXXXXXXX
250-DSN
250-ENHANCEDSTATUSCODES
250-8bitmime
250-BINARYMIME
250-XXXXXXXX
250-VRFY
250-X-EXPS GSSAPI NTLM LOGIN
250-X-EXPS=LOGIN
250-AUTH GSSAPI NTLM LOGIN
250-AUTH=LOGIN
250-X-LINK2STATE
250-XXXXXXX
250 OK
MAIL FROM: <email snip>
250 2.1.0 <email snip>....Sender OK
RCPT TO:<email snip>
250 2.1.5 <email snip> 
DATA
354 Start mail input; end with <CRLF>.<CRLF>
<email body here>
.

550 Requested action not taken: mailbox unavailable
QUIT
221 Goodbye
smtp reverse-dns spf

3 Answers

  1. You might consider making contact with the admins/postmasters of that domain. They may be able to provide more specific details as to what is triggering the rejection by their system(s).

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  2. You can use telnet to simulate your server. Try to see where in the dialog this comes up. It may give you a clue as to what is happening. Try sending to postmaster@OtherSchool, this mailbox should always be there.

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  3. Maybe their server doesn't expect to be contacted directly by your server - rather they are using a third party spam filtering service like Webroot or MessageLabs, in which case they will only accept mail sent via one of their service providers IP addresses.

    I've heard of this resulting in the 550 message after allowing all the preceding smtp transactions (it would be better if it just denied the whole smtp transaction to incorrect email addresses).

    If that turns out to be the case, then you need to figure out why your server is trying to send mail directly to their mail server.

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