avast! doesn't block anything, it just scans and reports if a virus is found.
I use MailWasher Pro with no problems, so there may be a conflict with 'pop waiter' I have no idea what this is?
I personally don't scan my MailWasher downloads as the content is only a small part of the email (to evaluate spam and no attachments) and is viewed in plain text. So I feel the risk level is minimal so I have added IgnoreProcess=MailWasher.exe to the [MailScanner] section in the avast4.ini file:
[MailScanner]
IgnoreProcess=MailWasher.exe
This excludes mailwasher activity from being scanned, even without this setting, it shouldn't block MailWasher just scan the email content being downloaded. So there is something else in the equation, perhaps your pop waiter?