I highly doubt that you have a virus on entourage. It's more likely that someone is sending email from their account, with your return address, effectively spoofing. Then you recieve the bounced emails that you never sent. I cannot find any reports of a Entourage virus on OS X.

I to am having that problem, do you know how I can stop the spoofing?

You can't really. The spammer's servers can append any email address they choose, so you don't have any control. However, you can send a polite email to the servers that are bouncing the mail asking them to stop it - as you're not really sending them. Usually the return addresses from the firewalls/servers are no-reply, so you'll often have to visit their website to get the webmaster's email address.

Hope this helps

I have the same problem! I tested my theory that Entourage was involved by changing my password (for the second time) but this time not using my iPhone nor Entourage to access my e-mail. I accessed it through my webmail server on the Internet and after a day of old kickbacks my e-mail address isn't being used for SPAM. My only problem now is how do I go back to using Entourage without starting up the SPAM problem again?

It is almost guarantee that you do not have a virus. The fact is that you can send emails that look like they come from anyone you want, it's great fun. SMTP and POP have no authentication of senders, there are protocols that do but their use is not widespread. If the message is actually coming from your computer then you probably should delete the application data of the app that is sending it.