I don’t bother to have avast check the small parts of the email downloaded to identify spam, etc. All email not deleted, etc. by mailwasher is scanned when it is downloaded form the mail servers.

Edit your avast4.ini file to exclude mailwasher, see below, find the [MailScanner] section and add the line IgnoreProcess=MailWasher.exe if there is no IgnoreProcess entry, if there is just add mail washer to it, second example.

This will obviously lower your protection very slightly (there are no attachments downloaded in mailwasher, nor is the small part executable when viewed as plain text), I feel the risk is negligible, the choice is yours.

[MailScanner]
IgnoreProcess=MailWasher.exe

[MailScanner]
IgnoreProcess=AnotherIgnoredProcessExample.exe,MailWasher.exe