Hi,
first thing, did you reboot after installing new avast! version?

please check following:

  1. in MAIL directory (typically in c:\Program Files\The Bat!\MAIL)
    should have been file AvBatEx.bav

  2. in the same directory in avconfig.ini should have been something like this:
    [Checkers]
    Count=1
    Checker #1=TAVDLLPlugin.C:\Program Files\The Bat!\MAIL\AvBatEx.bav

  3. if you have logging enabled, please send the log file to sedina@asw.cz

thanks! pavels