What do you mean Mail Shields, there is only one ?
What email client (web mail viewed through your browser or email program) are you using ?
Have you tried right clicking on the Avast tray icon > Avast Shield Control, does it show it is off there, if so try enabling it there.
If that doesn't work, try an Avast Repair - AvastUI > Menu > Settings > Troubleshooting > Repair App. Reboot after the Repair.