Telnet SMTP gives OK but not accepted or queued message and mails not received

The first question is where does the failure appear? Inside the exchange organisation or in emails sent outside the organisation? So the first question is does the error appear for internal emails, external emails, or both? OWA is obviously an exchange component in its own right, and so the emails are accepted by the exchange system the moment you click send, so we can assume that you've got a basic working system of some kind set up.

For external emails, as you say this is where the problem lies, I'd be looking at the SMTP send connector(s) for the organisation. You've got a box installed with the exchange hub transport role, and you might have another box configured as an edge transport server.

So the question is do you have an edge server? And what queues are showing issues? Are you using the default address space scope/ email routing? Are you using a smarthost? Is there a firewall between any of these servers? This is where I would start digging for the problem.

The unfortunate nature of current anti-spam techniques is that they are imperfect and prone to false positives if the recipient has settings wound too tight. I don't have a specific answer for you but I think there would be value in trying a few troubleshooting steps:

• Is this happening for all messages to that domain? Or just some?
• If the sender re-sends the message later, does it go through? (Just trying to determine if block is due to flow-control or is message specific.)
• Can you send manual tests (for example from the command-line via telnet) and successfully send a message?
• Did this behavior start recently? Or has this been an on-going issue?
• Have your users ever been able to successfully send to this domain?

If nothing obvious stands out, I'd suggest collecting a packet capture and examining the SMTP conversation. That might provide clues on when exactly the message got rejected. If not, you'll have some "evidence" you can provide to the recipient mail administrator who can hopefully elaborate on what's going on.

Hope this helps.