I turned all shields off for 10 minutes. I then turned them on a few minutes later and the mail shield will not come on. Meanwhile Thunderbird will not send or receive email. 30 minutes ago it was working perfectly.
Did you remove any (pre)installed av completely before installing avast ?
Reboot the system.
Have you rebooted, as simple as this sounds it often resolved issues like this.
If that doesn’t resolve it, try an Avast repair.