Entourage on Mac OS 10.5.8

[whdurham]

I'm using Entourage 12.2.4 on a Mac Pro 10.5.8 OS 2x2.8 Quad Intel. We can receive e-mail and reply to e-mail with no problem. However, each time we try to initiate an e-mail, the message comes back as undeliverable. This happens even when type in a name that is not in our address book. Does anyone have an idea what's wrong and how to fix it?

Thanks.
whdurham