Exchange 2010 outgoing email to certain domains not (allegedly) being received

Question

I have an Exchange 2010 server that hosts a couple of our business domains. Incoming mail is fine and 99% of the time outgoing mail is fine. I have had a couple of users claim that mail they are sending to only one or two domains is 'disappearing'. They send emails or reply to ones they receive, it goes into their outbox, and then goes to sent items as per normal. There is nothing stuck in any queues on the Exchange box... but the recipient doesn't receive anything from them.

I can't see any issues on our end and they claim they have no issues on their end, it's not caught in their spam filters etc. SO far I have two domains doing this - but everything else works perfectly, incoming and outgoing.

Just yesterday one of our users was emailing a client, replying to received emails and sending new ones... and halfway through the day the external client stopped receiving emails from our guy, even though Outlook said it was sent.

Any ideas?

I am getting no bouncebacks, delays or any other kind of error messages in Outlook or on Exchange.

Answer 1

From the command line of your Exchange transport server, initiate a telnet session to port 25 of whatever server the remote domain's MX record points to, and manually transmit a message to the recipient. If it accepts the message,but they do not receive it, this is NOT your problem.

EDIT: How to do this can be found here http://support.microsoft.com/kb/153119

Answer 2

Turn on logging on the Exchange server, it's a good idea to have it on all the time. Microsoft re-did logging facilities extensively and you don't need to worry anymore about log filling up your disk space. In order to do that, follow this and this article. You should be able to see exactly what's going on.