You need to specify another port for avast! SMTP listener (e.g. 26). Add this line to file C:\Program Files\ALWIL Software\Avast4\Data\Avast4.ini into section [MailScanner]:
SmtpListen=127.0.0.1:26
Then stop and start ashMaiSv.exe. Set the same port value for outgoing mail in account settings in your mail program.