If you use only MS Outlook (not Express), use msconfig to remove ashMaiSv.exe from startup settings.
If you use another e-mail program, insert this line
SmtpListen=127.0.0.1:26
to file C:\Program Files\ALWIL Software\Avast4\Data\Avast4.ini into section [MailScanner]
Terminate process ashMaiSv.exe and restart it.
Specify port value for outgoing mail (SMTP) as 26 instead of 25 in account settings of your mail program.
Edit: sorry, I missed the previous reply.