Extremely SLOW Mail Delivery

[onaweb]

Hello,
I am running Plesk 7.03 on a Windows 2003 Server. It has been taking approximately 4-6 hours for mail to be delivered using this server.

I even logged into the webmail (horde) and sent a message to myself and it was about 4 hours later that I received it. If I send a message with any of my other servers, the message is received in about 30 seconds.

The server currently averages about 5-10% CPU usage.
Here are my stats:
CPU
Last 1 minute 8%
Last 5 minutes 2%
Last 15 minutes 1%

Physical Memory Usage 55.55%

Virtual Memory Usage 20.48%

Hard Disk Usage 11.31%

What can be causing these extremely slow delivery?

Thanks for any help you might be able to offer.
Andy