I have been chasing down a different issue, and came across this:
Every time i try to exit an extension, i see the following log pop up on /var/log/asterisk/freepbx.log
[2020-Jul-23 14:18:40] [freepbx.INFO]: [NOTIFICATION]-[fax]-[invalid_email] - Invalid Email for Inbound Fax (User Manager users ‘…’ have the ability to receive faxes but have no email address defined so they will not be able to receive faxes over email,)  
I’ve played with the system and watched the logs enough to verify it is the act of editing any extension that causes this log to pop up… but WHY? i dont see the correlation / reason why this should be the case.