Please insert the line
Log=20
to file C:\Program Files\ALWIL Software\Avast4\Data\avast4.ini under [MailScanner] for detailed logging.
Then send one mail (or retrive mail if incoming mail is the problem), which will generate the log file. After that, send me by mail the log file C:\Program Files\ALWIL Software\Avast4\Data\log\aswMaiSv.log. Thanks.