You may very well have a real problem here. Today, I noticed that I was getting a strangely small amount of email. I logged into my primary ISP's webmail site to discover over 100 pieces of undelivered mail. Entourage got it fine. Now, I am trying to figure out what I have done over the past few days to precipitate this turn of events. The only thing that comes to mind is that I downloaded and installed the iCal update via Software Update. Strangely, Mail seems to send mail from this account without problems.
Update: I may have just "fixed" the problem. My sent an email from my primary account to my primary account. Sending had not stopped working. However, I just received the email that I sent to myself.