Exchange 2003 - POP3 Users can't send; no such domain...


Somewhat eXPerienced
24 Dec 2001
Hey guys,

I'm having a huge problem when it comes to attempting to send emails from our network and the POP3 users. This has been an ongoing problem that has peaked, and is basically shutting down the municipality's ability to send email outside. They can send emails back and forth inside, however outside just fails. When users attempt to send mail, they get the following error message:

The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was ''. Subject 'Test', Account: '', Server: '', Protocol: SMTP, Server Response: '550 No such domain at this location (', Port: 25, Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC79

That is the error message, with some changes made to account for security. My co-worker and I are banking on that it has to do with the authentication being made by the users. The users are setup as INETORGUSERS so that they can use POP3 (they're on a different network than the Exchange server; 2 networks, 1 building). They set the POP3 to connect to the server, and that they have to authenticate before being able to send. But now for whatever reason we get that message as soon as we hit send / receive.

I have verified that the firewalls on both sides aren't denying the connections. We've also confirmed that the new spam filter we've implemented (Barracuda) is not doing the outbound scanning and should not be interfering. So what's the problem? The only thing I can think of causing problems is that before we implemented the spam filter, we ran about 25 updates or so for the Exchange Server.

Any ideas?
well.. given the fact that POP3 is used to retrieve mail and not send mail, that's your first problem. Second, port 25 is the SMTP port and IS used to send mail. However, if you are trying to use SSL over SMTP, this is usually done over port 465.

All that being said.. what email client are they using? Have you allowed them to relay off your mail server?
Well, the POP3 users are the ones' having the problem, that's why that was mentioned. The server is setup that as long as they can authenticate, they are able to relay mail through the server. All clients (Mostly Outlook 2003, some Outlook Express) are set to authenticate using the incoming server credentials before sending. They have all been set to port 25 previously and are not using SSL.
Resolved! Turned out it was the Barracuda device, by default it disables any kind of email relaying out of the box. We left most of the default options enabled on it. I spent more time on the phone with Microsoft than I did on the phone with the technician; enabled relaying for everyone sending from the domain and all is well.


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