Some people set their BT client to use one of the privileged ports (25, 143, 80 etc.) to avoid ISP blocking, or firewall restrictions, but this can cause problems with local proxy engines, like the one in avast. Most BT programs have an option to prohibit connections to peers that use such ports. You can set Vuze to not connect to such peers, or perhaps some of the developers can tell us how to exclude a specific program from mail shield scans?