Does your new computer come with a shiny new antivirus that is causing contention issues as it scans the same file as Thunderbird is trying to write to, my guess is yes.

Does that antivirus come with a VPN? Again I would guess yes as GMail/Google hates them and folk who use them have lots of issues. You have issues with gmail. Have you tried to send mail from your Comcast account. You probably can't. Again the VPN. as you do not appear to be connected to the internet via their service and they do not permit mail being send unless you are.

Thanks for your response.

We recently switched from Comcast to Verizon fios. Much to my surprise Comcast allows you to keep using your Comcast email even after you've canceled your service. The only requirement is that you log into those accounts in their web UI once every 90 days. So for the time being we have no issues sending and receiving Comcast email despite not being on their network. I'm not counting on that lasting forever. It does make transitioning to gmail easier.

I use a VPN for work but I do that inside a virtual machine. I do not use any form of VPN on the host machine. The issues happened whether the virtual machine was running or not.

The new computer did not come with anything preinstalled but we have been using Bitdefender for some time now. At the first difficulty I uninstalled their Thunderbird plugin and that didn't seem to make a difference. What has helped - and possibly solved the issue - is to create exceptions for the two gmail mailbox folders in question. Makes some sense though we don't need to do that for Comcast and never had to do it on the old computer. Or on my wife's machine for that matter. Go figure.

But it has worked since I made that exception.

Again, thanks for responding.