Steps to reproduce:

Run Thunderbird (102.3.0 32-bit or 64-bit).

Actual results:

Mail and Contacts/Calendar sync starts, shortly thereafter all network communications stop. Machines are new (10th Gen Intel) and old (Intel NUC 4th Gen), both Windows 10, both become "Not responding" during attempting to update email folders. Eventually mail server timeouts happen (different accounts, different mail servers, same results). The only thing that will not stop is the Contacts/Calendar sync built into the Address Book and Calendar (on the new machine, old one doesn't have any syncing).

Email will come in, after roughly 10 timeouts, only receiving small emails once every 15 minutes, sending emails during this time fails 100% to send. 102.3.0 introduced this issue.

Expected results:

Expected results is the downloads don't timeout.

Try windows started in safe mode

Both machines are limited to WiFi, neither of which work in Safe mode, drivers refuse to run. This wasn't an issue prior to the 102.3.0. The one machine is new (as in new new, Windows 11 ready), worked fine up until 102.3.0, only thought is it could possibly be a bad upgrade (but on more than one machine, that would mean something worse).

Guess: firewall (or AV) issue.
Which version did you upgrade from?

Facing the same issue on Win-10 after the recent update to 102.3.2 . This was not happening earlier on 102.3.0
Mail server timeouts are happening, and mails are not downloading completely.

Try disabling AV/firewall.

Windows AV/firewall, tried both off, made no difference, it isn't Windows causing this or any router, the software simply goes "Not responding" for a good 30 seconds on a new 10th gen i3 machine, please, stop blaming the machine.

I have heard some reports of calendar slowing things down to the point of hanging. Do you have a remote calendar? Try disabling it.

(In reply to mr.armh from comment #6)

Windows AV/firewall, tried both off, made no difference, it isn't Windows causing this or any router, the software simply goes "Not responding" for a good 30 seconds on a new 10th gen i3 machine, please, stop blaming the machine.

Questions are asked because it is part of the process of elimination. If the software simply goes "Not responding" then in most cases the culprit is something intervening or scanning eg: Anti-Virus or Firewall or AntiMalware or something else is updating etc, maybe a conflict between the program and hardware in the computer, lack of system resources, Internet connection is extremely slow or software bugs.
It will help you to test for issues in general not just Thunderbird, if you invest in an ethernet cable and it is not costly.

These types of possible issues need to be ruled out.

One may be a new machine, but that is not a guarrantee the drivers are up to date. Are your drivers all up to date? Check all the drivers.

What other programs are running? Drop the load on the Cpu by exiting programs or tasks.

Open the 'Task Manager'. Start Thunderbird. How much CPU & Memory is being used by Thunderbird? It may start high and then drop. It may say 'not responding'. Some images showing what you see at these various points may be helpful.

(In reply to Anje from comment #8)

(In reply to mr.armh from comment #6)

Windows AV/firewall, tried both off, made no difference, it isn't Windows causing this or any router, the software simply goes "Not responding" for a good 30 seconds on a new 10th gen i3 machine, please, stop blaming the machine.

Questions are asked because it is part of the process of elimination. If the software simply goes "Not responding" then in most cases the culprit is something intervening or scanning eg: Anti-Virus or Firewall or AntiMalware or something else is updating etc, maybe a conflict between the program and hardware in the computer, lack of system resources, Internet connection is extremely slow or software bugs.
It will help you to test for issues in general not just Thunderbird, if you invest in an ethernet cable and it is not costly.

Yes, I understand that, there is no extremely slow system involved in either case.

These types of possible issues need to be ruled out.

One may be a new machine, but that is not a guarrantee the drivers are up to date. Are your drivers all up to date? Check all the drivers.

Both machines have 100% up to date everything (including firmware), I should know, I manage them.

What other programs are running? Drop the load on the Cpu by exiting programs or tasks.

Standard Windows 10 Home services and Thunderbird in both cases. Task manager showed 1% CPU usage on the new machine and 14% CPU usage on the other (due to DWM doing remote).

Open the 'Task Manager'. Start Thunderbird. How much CPU & Memory is being used by Thunderbird? It may start high and then drop. It may say 'not responding'. Some images showing what you see at these various points may be helpful.

The new system CPU was at 0% instantly and Memory sat around 150MB, not sure if it spiked, due to how fast it loads and runs (M.2 and the 10th gen i3), the older one's CPU was at 5% then 0%, Memory was slightly more at around 160MB, but it was a bit slower so response on the load spike would have been unavailable.

In both cases, calendar and contact sync were being done (by Thunderbird), the new one was showing that it was still trying to sync contacts, that never went away (Activity Manager).

Not sure when this issue crept in, but pre-100 versions do not exhibit this.