Hi Jeff,

Was Thunderbird running well until recently? What else changed on the computer around the time that it stopped running well?

One possible cause of the problem is your anti-virus software. In almost forty years of using Macs and following Macintosh news closely, I have never found a reason to use anti-virus software. Well, maybe to defend against malware in Microsoft Word macros for a while. Apple includes anti-virus functions in its operating systems.

Another possible cause is your VPN. I use Mullvad VPN and never have a problem running applications. Some users seem to have problems with other VPNs, and some support people on this forum frequently advise turning off the VPN. If you're using something other than Mullvad, you might want to consider switching.

In your place, I would first disable the anti-virus software and see if the spinning beachball persists. If it does, I would leave the anti-virus software disabled and turn off the VPN, and see if the spinning beachball persists.

I can also imagine a memory problem. Does your Mac mini have an Intel CPU or Apple Silicon? It's more likely to run out of memory on a machine with an Intel CPU. What else is happening on the computer when the beachball spins?

Is there something more to the description of the problem? How do you know that sending and receiving e-mail messages do not work if all that happens is a spinning beachball that makes you force quit? If you get far enough to try sending and receiving and neither one works, then that is a different problem to work on  … 

Please report 1) your incoming server settings and 2) your outgoing server settings. Incoming server settings are in account settings > server settings. Outgoing server settings are in account settings > click on account name > edit SMTP server. You can get to account settings by several routes, including 1) right click on the account name in the folder pane and select "Settings" and 2) click on the ≡ button at top right and select "account settings".

We will need this information for each server:

server name: port: connection security method: authentication method: username: just report whether you are using your full e-mail address or just the part before "@" or something else

Does your e-mail service provider require an app-specific password, a.k.a. security password or app password, for third-party e-mail clients such as Thunderbird?

If this problem is not solved quickly, you could access your messages through webmail or by importing them into Apple Mail.