It was working fine and then it broke, with error "Unable to connect"

What is your operating system? I will assume Windows

What is your Anti-Virus, if any?

Do you use a VPN?

"Localhost" refers to your local computer. Normally Thunderbird interacts with the Internet directly. There should be no local proxy involved unless you intentionally use one.

This can often be caused by an Anti-Virus program inserting itself between the mail agent (Thunderbird) and the Internet in order to "helpfully" scan your emails. It could also be caused by malware doing the same thing for less "helpful" reasons.

Some things to try:

- Check the Thunderbird setting <Settings | General | Network & Disk Space | Connection> and click the "Settings" button. The default setting is "Use system proxy settings". A clue could be if "Manual proxy configuration" is something you don't recognize. Try temporarily changing it to "No proxy" and restart Thunderbird.

- Turn off your VPN, if you use one.

- Temporarily disable your AV or disable any of its options to scan your email.

- Restart Windows in Safe Mode with Networking. This temporarily disables the AV.

Please report your results.

Well, that didn't work! I do have Windows 11. My anti-virus is PCMatic. I don't use a VPN. I did all of the "Some things to try" with no success! I can't find any settings in PCMatic to scan email. I don't know how to restart Windows in Safe Mode. I will try to figure that out!

What happened when you tried to change the proxy setting?

I feel certain that it's PCMatic that is the problem here. Anything that forces Thunderbird to use Localhost is an external program, not part of Thunderbird. I notice that they have a bunch of service features and offerings, including a firewall, VPN, ad-blockers, etc. I'm guessing there's a mail scanning feature buried in there somewhere. I don't know anything about the program, so if you're having trouble controlling it, I'm afraid you'll have to check with their tech support or forums..

Restarting in Safe Mode with Networking should disable PCMatic temporarily. There are many descriptions on how to use Safe Mode - here's one (I can't vouch for it being the "best" or most understandable.)

https://windowsforum.com/threads/how-to-boot-windows-11-safe-mode-all-methods-and-tips.385256/

I did all of the proxy things you told me and I restarted in Safe Mode but Thunderbird still doesn't work!

Well I changed my email password for the umpteenth time and then Thunderbird decided to start working! How do I get out of Safe Mode? Or do I need to do anything?

Normally, Windows Safe Mode is temporary, that is, when you reboot after that you'll be back in normal mode. But if you're still in Safe Mode right now, and Thunderbird is connecting OK, that's a sign that your AV, PCMatic, is what's interfering. If, after you reboot back in to normal Windows mode, and you find out that you can't connect, you're going to have to find out how to configure PCMatic to stop messing with your email connection.

But you might get lucky and it all works as it should. Please let us know how it goes.

ACCESS DENIED is back! It was working fine until this morning!

I'm convinced this is a PCMatic problem. Maybe this article can help you tell it to leave Thunderbird alone:

https://documentation.pcmatic.com/article/191-allowlisting

That must be an old article because I finally found the page I needed on PCMatic, and Mozilla Thunderbird is marked as ALLOW! This is so frustrating because I used PCMatic on my old laptop and had no problem! Very strange!