The Mail shield counter does nothing here too, but if I keep the statistics page open and send and receive an email to myself, I see the two entries appear in the last scanned items.

I’m also using Thunderbird (24.1.0) with incoming connection security SSL/TLS and outgoing None (dictated by my service provider).
Mail Shield settings - All three boxes ticked.
OS Window 7 Pro, 64 bit
Avast 2014.9.0.2007.