If you could supply the error messages that accompany these issues perhaps someone could guess, but the description you have so far could just as easily describe a visually impaired person walking into a fence. Something keeps stopping them walking forward, they have no idea what it is, but they know they can not go there.

(That person was my brother-in-law by the way. I am not making it up.)

I can say that certain antivirus products make a habit of preventing mail connections, so do some VPN's with some providers (many mail providers will guess you have been hacked if you use a VPN and raise a flag/alert instead of letting you in.) Others lock account for sending spam, or to many wrong passwords (that was a bug in the not so distant past where Thunderbird would retry a failed connection and the provider would disable the account)

But really I am just offering some suggestions off the top of my head because I have no specifics to work with, but by far the most likely cause of a failure to get mail after an upgrade/update will be your antivirus/ Software firewall.

Hi Matt,

Thanks for your thoughtful response. The error I get says, Login Failed. There's additional stuff about not being able to log into the server. I'll have to wait for the problem to recur to get you the complete message unless there's a log file hiding somewhere. Do you know?

By the way, it's working fine today and I changed nothing.

Will post an update next time it happens.

This is an ongoing problem. Yesterday, everything worked fine. Today, none of my accounts (two Yahoo, one Gmail) is able to log into their respective servers. Here's the error: