You can increase the scanning limit, see
http://forum.avast.com/index.php?topic=11788.0#msg100032
Or you can edit avast4.ini, find the section [MailScanner], add (or edit) the line IgnoreProcess.
[MailScanner]
IgnoreProcess=newbin’s_process_name.exe
of course, changing newbin’s_process_name.exe for the real process (not file) name.