In the mail shield, expert settings, Program Behaviour, General and it allows you to insert a note into clean messages (that is what I meant by insert a clean note), e.g. this message has been scanned by avast and is clean, etc.
So they are not inserted by default, So my comment is based on if you had changed the default settings and as a result caused this affect "question marks and a few other signs."
Yes, it is a bug if in enabling that option is causes a problem and hopefully they will get it sorted in the MS Outlook plug-in as this problem isn't in the standard Mail Shield when used on other email clients.