Thanks, and I'll do that.
I only started using Edge a month ago after using Chrome for years, and just tried firing up Chrome and using Gmail and there's no issues there.
From the looks of it then Gmail doesn't need any browser synch settings since it synchs itself whenever a new email arrives or when it's trying to connect after being blocked (which is the extremely annoying problem I had yesterday at first).
The problem on Edge has reduced in frequency and now only manifests itself when I delete an email, whether I've read it or not.
I can open and read and then close any email without a problem so it's Edge specific and only that action that triggers the alert now.
This should be enough info for Avast to try and get to the root cause, but since it's only me having this problem then it seems like it's specific to me for some reason.
Anyway, will send the report now and thanks again!
Cheers, Will.