You have to check the bittorrent settings and stop it using email ports for communication or file transfer as that will trigger the Mail Shield to scan the content, which will fail for two reasons, 1. it isn't using email protocol and 2. it is using a secure connection.
I don't use any p2p application, so I don't know what settings to use or how you would stop it from communicating or transferring files on ports 25, 110, 119, 143 which avast monitors.