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SMTP 550 relay error when sending large attachments

"SMTP 550 relay not permitted" errors can occur when sending email with large attachments. Find out how to telnet to port 25 to test SMTP communication and stop these relaying errors.

Our Exchange Server 2003 Enterprise Edition (running on Windows Server 2003) works fine in terms of sending and receiving email -- except when we send large email (e.g., mailshots or email with large attachments). Incoming email bounces back to the sender with the following error:

Final-Recipient: RFC822;
Action: failed
Status: 5.1.1
Remote-MTA: dns; relay-2.mail.demon.net (
Diagnostic-Code: smtp; 550 relay not permitted

Users connected via Outlook Web Access (OWA) also get disconnected.

Our ISP has set three MX records, one pointing to our IP address (which forwards to our Exchange server) and the other two pointing to relay-1 and relay-2, which are supposed to be backup servers at DEMON (our ISP).

We have a very slow connection (512/256 KHz), but it has always worked before.

All bounce back email shows that relay is not permitted from either of our SMTP relay servers.

I looked at the MX records associated with your domain and tried to connect to port 25, and all three MX records timed out. I'd suggest that the issue is related to your slow link timing out on large attachments. Take a look at some of the recommendations discussed in this article on how to use telnet to test SMTP delivery from outside your network.

Do you have comments on this Ask the Expert Q&A? Let us know.

Related information from SearchExchange.com:

  • Expert Advice: SMTP error 550 5.7.1: User unable to relay message
  • Tip: Many ISPs now blocking port 25
  • Tutorial: A primer on DNS and MX records
  • Reference Center: SMTP tips and resources
  • Dig Deeper on Exchange Server setup and troubleshooting

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