Mailwasher works just perfect with avast! They love each other, actually they adore each other...
No problems at all. There is nothing you have to configure in your Mailwasher except your pop3 server names, login name and password for each account you want to be checked. After that, just run Mail Protection Wizard (it's in your START menu / Programs, under avast@ antivirus... Let him autoprotect all your accounts and it will work just like it should work.
Avast! must change your pop3 and SMTP server names into 127.0.0.1 in order to take over over your mail client... otherwise, no point to use antivirus mail protection, right ? Same story with account name: yourname@youraddress.com#pop.youraddress.com
In avast! On-Access Scanner, did you start INTERNET MAIL PROVIDER ? If not, you should do that, 'cause that provider is for Outlook Express and all your WEB mail related accounts. If you don't use MS Outlook/Exchange (big difference between Outlook and Outlook Express), you can disable OUTLOOK/EXCHANGE provider without having any problems...
Cheers !