Eudora Email Virus???

My father has Win. 98 se and is using Eudora. He can receive email but he can’t send any. Is this a virus, or is this a glitch in Eudora? He has been using this program for 3 yrs and has had no problems until now. He said he has scanned his pc with 2 different anti-virus programs, but they are not showing anything.

Any ideas as to what is causing this?

Thanks!

Karen