You didn't mention any error messages so I assume the email is not exceeding your mailbox limit.
I had the same problem and traced it to the latest/greatest USR WinModem. A WinModem does not have a processor and relies on the main processor of your motherboard. "Timing is everything"... sometimes the WinModem, motherboard bus, and motherboard processor get out of sync, a data packet gets missed which then results in a standoff or 'time out' of the operation.
What really hacked me off was USR didn't state on the box or in the included documents that it was a WinModem, I finally discovered the true classification of the modem buried real deep in their online troubleshooting site.
If you do in fact have a WinModem, try swapping it with a regular Modem and see if the email will deliver. If so, you know what to do with the WinModem.