See the below post to disable Avast Inbound Mail scanning.
Note:- If the below post setting is not the same as your version of Avast, then suggest to go to the Avast Forum for support on that product. Avast WEBforum - Index.
eM Client 8 Avast work-around - Mail - eM Client
Hi,
Avast work-around
Protection > Core Shield > Settings (gear on the right) > look for Mail Shield > disable Scan inbound emails
eM Client 8 should work
Hope these helps.