... forgot to say that Send works. Everything but Get Messages works - neither method of doing a Get Messages works ("cloud button" nor "File -> get new messages for".

I is funny to me that I have been responding to users of McAfee, Norton and Kaspersky in this forum for more than 10 years with exactly the same issues. Mail is broken. Your antivirus is most likely broken, and in the case of Norton it even lies about being disabled. Restarting in safe mode of the operating system is the only way to really get Norton to actually let go, or uninstall it.

Restarting your machine will probably fix the issue for a while, until Norton nods off top sleep again.

Matt,

 I finally got it to work - by uninstalling Norton 360 Premium.  I don't want to run without some form of

active AV program. Running Win 11. You don't seem to like any of the AV programs I know ... so what do you suggest? Should I just re-install Norton and "hope for the best"?

I use defender. It is non intrusive, comes with the operating system and avoids the alarmist messages I am used to seeing from the third party market. Something to consider is that these products entire marketing strategy is fear. Fear of being hacked, fear of loosing money and data, fear of the unknown. This is exacerbated by most folk not really understanding the whole "infection" thing as to what it really means.

It is to me noteworthy that email scanning is not something that Defender does. When I put the question as to what email scanning does to representatives of antivirus vendors the best they could offer was "another line of defense". The French learned in 1939 just how lines of defense work. The German army just went around the end of the Maginot Line. I see email scanning in the same light. Why send an infected email when you can send a link in an email that will achieve the same result, only better as web browser routinely allow scripting and drive by downloads of data packets.

An email can not hurt your system. It is a plain text file. What can hurt your system is what your email program does with that plain text file when it displays it. In the case of Thunderbird I see it as fundamentally safe. I am sure there is some risk, just as there is some risk in breathing. But when using Thunderbird no script in the message body will be run, no remote images will be shown (unless you have changed the default). This makes it very different to almost all desktop mail clients. Outlook and Outlook express ran everything and started the urban myths that surround email and what to be cautious about and when to avoid and above all do no open suspicious emails. I open suspicious email all the time in Thunderbird. It does not matter if the email body has some malicious script embedded in it, because Thunderbird will ignore the script. Even the HTML that is used in the email is sanitized by Thunderbird. Not all HTML tokens are implemented in email bodies. Things like iframes just don't work for example. It is why the links folk try and post in email for YouTube videos are very dodgy in their display in Thunderbird, let alone implementation. They use an embedded iframe always and sanitizing the HTML in an iframe is just to hard, so we don't allow them.

Attachments, the primary source of infections in email in recent years simply do not exist in the emai source. At least until you try and save or open them. Until then they are encoded in the only way they can be included in email, as plain text. In transmission all attachments are encoded using MIME to make them text as email is a text based protocol.

You see a word attachment in the email but it really does not exist as such. Looking to the message source there is no binary file, only some MIME text that is defined to be an attachment. You select to open the file or save the file and Thunderbird has to decode the text and convert it back into it's original format. At this point the file will appear in the systems memory, or the system temp folder and any antivirus worth having will spot the virus if it contains one and prevent the data being executed. It really is that simple and the primary reason Defender does not scan mail in my opinion. It will detect the threat another way when it actually becomes a thread instead of quarantining your last 10 years of mail in your inbox because you have received a latent threat.

There are other volunteers here that use Norton. Toad-Hall being one. Personally I just don't have the time or patience to be treated like an idiot by software that is not entirely truthful about what it is doing and when. Over the last 20 years I have cycled trough Nortons (they did not disable when they said they did), AVG (Which kept flashing annoying icons on the task bar because I disabled the email scanner, McAfee (It came with a Dell computer and was just the pits. But I had already seen it here in lights since 2012 so I had little patience. I think a week.)

Then I was with Nod32 for a number of years until I purchased this machine a couple of years ago. By then the 5 license Nod package from ESET had become quite expensive each years and I no longer had the Windows server it was specifically purchased to protect. Added to that ESET had released more consumer oriented product with the usual alarmist messages. SO I decided life was not long enough to put up with the new ESET

When this machine was new I read some reviews and defender stacked up pretty well in the primary task is blocking malware. So I bit the bullet, as there was little on the new machine that would be lost if defender turned out a dud, and I went with the Microsoft package. Sure it does not shred my deleted files, it does not tell me my passwords are safe in some vault and then merrily offer out of date passwords until your account is blocked because it messed you changing the password. IT does not ask you to allow it to be an encryption certificate issuer as it does its scanning locally. Most importantly to me it does not get in my face unless there is actually a real tangible problem.

So I will not recommend anything in particular to you, sure reinstall Norton. Once you know it fails dismally occasionally on POP mail retrieval resulting the the same mails being downloaded over and over, tends to block new versions of Thunderbird getting mail and it's other foibles like requiring you to add a self signed certificate to Thunderbird certificate store to allow it to hack your encrypted communitcations. It is relatively easy to live with. The regular maintenance is really no a major issue, once you know what it is and what to do. Personally I would just rather not.

While doing your research have a look at this link https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues

Some of those issues are more than 10 years old and the page gets few updates because in reality all products are basically as bad or good as the next. Many even come from the same parent company, for example Gen Digital Brands: Norton; Avast; LifeLock; Avira; AVG; ReputationDefender; CCleaner

Matt,

 Thanks for the lessons.  I'm not new to this stuff - but I still learned some things.
 I got impatient and said "now that I know what the cause is (Norton) and the fix

(uninstall Norton) - I decided to try reinstalling Norton. So far "no problems".

 So, I'm gonna leave it.
 I'm not sure but I think I was seduced by another of those Norton "safety prompts"

to add something or other (probably a "plug-in" of some type/for some purpose). I've been leaving those off and so far everything is working the same as it was before.

 Thanks again.  BTW - getting into safe mode in Win 11 is non-trivial and tedious

(typing in that long "key" ... more than once) and something that is likely to send the novices running to a paid tech support.

                                                                          - as I said ... Thanks.