thanks catabotz,
that’s a temporary Solution. I tried a couple of Fileexclusions, which
all (except *.html ) have no Effect , but *\nsemail.html works and
seems to be secure enough in aspiration of solving this Problem
at the root by Avast in the next days/weeks.

@Maxwell Systems
I won’t hope , that the Mail Promotion Attachment has something
to do with this Problem.