You must change your email account settings into Outlook Express to use another port than 110 (pop3) or 25 (smtp).
You can download and use TCPView application (see Links on my signature) to see which application is using each port.
If you set Eudora to use 111 (for instance), you can set avast4.ini file (see Settings on my signature) on section MailScanner.
Hope this help.