What is your firewall ?
Do they allow ashWebSv.exe (web shield) and ashMaiSv.exe (email scanner) ?
The latest version of MWP is 5.2, shouldn't cost to update ?
I also use MWP and I have it excluded from avast scanning.
By editing the avast4.ini file, [MailScanner] section using a text editor like notepad. It is best to save a copy of avast4.ini to another location in case of any problem, you can then copy the original back.
MailWasher doesn't download the complete email to do its analysis, it only downloads the headers, a small part of the body, it doesn't download images or attachments and it views what is downloaded in text only. Based on this I personally don't feel that any negligible risk worth scanning duplication, but the choice is yours.
[MailScanner]
IgnoreProcess=MailWasher.exe add this line if you don't already have an IgnoreProcess line.