Hello,
I use avast v11.1.2245 on win8x64 pro and have this issue:
Open: Mail Shield → Customize → Behaviour
Uncheck “Insert note into clean message (outgoing)”
Click Ok to close Mail Shield Settings
Click Ok to close Settings
Open Mail Shield → Behavior again
Expected: checkbox “Insert note into clean message (outgoing)” is unset
Actual result: checkbox is set
Is it bug or feature?