Ok ... first the small print. I am an avast user just like you and I have not tested the following but ... as a very supercilious but wise older lady told me many years ago (with a more in sorrow than in anger tone) ...
"Alan ... it usually works for those who read the instructions."
So, I am reading the instructions and I must also assume that you are following the recommendation of the avast team to use the Outlook/Echange provider for Outlook which, for Outlook users, is far superior to the Internet Mail provider.
In the Outlook/Exchange provider:
Customize > (Use the arrow pointers to get to) "Heuristics" tab > Move the Sensitivity pointer to "Custom" level > Use the arrow pointer again if necessary to) "Heuristics - Advanced" tab > Change the "Total count" box to a number greater than the total number of your addressees > "OK"
I hope that helps - please let us know.