edit avast4\data\avast4.ini file, find the section [MailScanner], there shoud you find

[MailScanner]
PopListen=127.0.0.1:110
SmtpListen=127.0.0.1:25
ImapListen=127.0.0.1:143

change it to some other values, I recommend
[MailScanner]
PopListen=127.0.0.1:12110
SmtpListen=127.0.0.1:12025
ImapListen=127.0.0.1:12143