Sounds like firewall blocking.
Is that MS Outlook (not express) ?
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.
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.
[MailScanner]
IgnoreProcess=MailWasher.exe add this line if you don't already have an IgnoreProcess line.