delayed delivery problem

Hi:

Having a problem and not sure where it is:
We are running IMAIL v8.13 on windows2000 server.

Incoming email works fine.

On occasion, SOME outbound emails can take 12 hours to be delivered. There are no bounced/delayed/retrying messages and other email is delivered within seconds. This happens for mutliple senders and multiple recipients. The emails are small text messages with no attachments. Nothing unusual in the log files.

Tech support says the mail server is functioning properly and the ISP reports the DNS settings are OK.

We have plenty of bandwidth and have no other Internet related problems.

What could cause such a large delay in outbound email?

Thanks