Personally I would have thought that to have your anti-spam filter the traffic first, delete the crud from the email server before downloading the remnants. That way avast is unlikely to catch what might have been filtered out.
I use MailWasher Pro, although it is primarily for Spam it is also easy to deal with suspicious emails. There is a free version, but this only works with a single email account. The Pro version works with multiple accounts.
Other than that you only have the option in the Internet Mail provider, Customize button, Advanced tab, Silent Mode, With General answer No.
I can't recall if that deletes or sends to the chest (effectively you can't subsequently restore from the chest).