See your avast4.ini file. You will find in the section [MailScanner]
DefaultPopServer=
DefaultSmtpServer=
with this information. Sending a file for analysis from the Chest, avast! will use this smtp server, for instance.
This information in under each email account configurations and are handle by the Mail Protection Wizard. avast! gives support for multiple smtp and pop servers (i.e., you can use more than one).
Did I make myself clear? :