usually cause by turning off the use less secure apps setting on the gmail web site during one of their security checkups. Did you do that?

I have changed no settings on my Gmail. In fact I have two-step verification so if I understand correctly that doesn't apply here. I have Thunderbird set up on two separate computers, my office computer and one of my retail computers out front. It's the retail computer out front that is having the problem. I have done nothing to either of them but the office computer still receives and sends perfectly.

This, by the way, is a recurring issue on the front computer. It crops up and goes away on it's own but is a pain while it happens.

Still struggling with this. Thunderbird will not download gmail at all.