That is what Igor suggested, but personally I would do without the Internet Mail and continue with The Bat plug-in. I think that the insert clean note into clean messages is of little value.
I don't trust any note that states that 'this email has been scanned by X anti-virus,' avast scans all emails and many other AVs would also scan all emails regardless of origin or if it had been scanned, assuming it could determine that. Yes it may give a warm fuzzy feeling but it doesn't mean anything, there are even malicious emails doing the rounds that have a similar note that this email has been scanned by X anti-virus, don't take anything at face value.
The integration of the avast plug-in in The Bat is much more useful than any clean note, it allows for the scanning of secure email, something that the Internet Mail provider can't do without a 3rd party interface like STunnel.