@Lynda_Carol_Kelly Here is another thread post (variation) on disabling Avast scanning of incoming mail as @skybat advised above.
eM Client 8 Avast work-around - Mail - eM Client
WKeat
Jul '20
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.