What software generates the SMTP bounce message "550 Envelope blocked - User Entry"?

Question

I'm troubleshooting an issue where email sent from a specific user is blocked every time to only a specific sender. The NDR generated (from our Exchange server) has the following message:

#550 Envelope blocked - User Entry ##

The recipient says that the sending address is not in the blocked senders list. The recipient can also receive mail sent from other addresses sent from our MTA, and other users at the recipient's company can receive mail from the sender in question.

What I'm really trying to figure out is what software in the mail flow might be generating this specific message. Web searching hasn't come up with anything yet. Thanks for your help!

Solution

Any software can generate such a message. You can configure almost all mail servers to generate such a message; even on a recipient or sender basis or content basis.

You have to ask this question to the recipient (or better it's postmaster). But you can try to identify this on your own. Therefore you need the IP of the host that rejected the mail. Try to telnet <ip> 25 and you can see the server greeting of that host. In many cases it also displays the server software and it's version. But this can also mislead in the wrong direction as even a before-Queue-SMTP-filter in front of the real server can generate this message.

Whatever software the recipient uses it won't help you to know. You have to discuss this with the postmaster of the recipient's server. Nothing else will help.

Comments

[Ryan Ferretti]

The headers should tell you the path that the message took on it's travels and it should say who through the error.

[JdeBP]

It seems fairly clear that M. S already knows who, and is asking whether xe can determine what software that "who" is using, to gain some clue as to where this restriction might have been configured, given that the final recipient doesn't seem to know. (Obviously the final recipient and xyr own postmaster are not on speaking terms. ☺) The correct place to be looking is not message headers but SMTP Relay server greeting banners.