You could run the Mail protection Wizard and change it.
You don't say what OS you use, this is for XP, windows Start button>All Programs>Avast>Mail Protection wizard (the Help file can also be opened here).
Or you could change the avast4.ini file located in the C:\Program Files\Alwil Software\Avast4\DATA folder.
Reading the help file "Setting up the Mail Protection" section should also help.
HTH David