This is not tbirds problem - it apparently the pop servers having some limit on connection time. Further, apparently you go to the pop server and ask for an increase. I have no idea having never heard about it but someone else here may.

Thanks. What is strange is that the problem appeared suddenly today while the account worked smoothly for about two years with the same settings.

it is also almost always the result of an antivirus making things take to long. Do you have one scanning incoming mail perhaps. Or encrypted internet connections. Or a VPN which can cause all sorts of random issues with email ports/ connections with providers.

This link has a fairly good and simple to follow discussion on that happens when you attempt to get mail using pop. https://www.atmail.com/blog/pop-101-manual-pop-sessions/ In there they say that "STAT returns the number of messages in an inbox and their collective size (in octets)."

STAT
+OK 2 13410

In this case, 2 messages totalling 13,410 octets (or more commonly, bytes).

That this command is failing would indicate your authentication has failed or timed out. (that antivirus,VNP idea), without Thunderbird being aware this has occurred. To be sure you would need to log the evets and report the information in the log. To enable this you would use the Config Editor to modify the hidden preference mailnews.pop3.loglevel, to All.

After you attempt to get mail and fail, open the error console( Alt+Ctrl+J) to view the errors and from there you can copy then and paste into a reply on the forum or into a text file. But before trying to get details, I would simply disable the antivirus scanner to see if it is the problem as that is the easiest thing to do first.

Many thanks indeed. The issue broke up suddenly on 3 different computers suing this same account. Two with TB, and one with Postbox. Before this, the behavior was correct for two years. Moreover, none of these computers use a VPN or an antivirus for internet connection. Nevertheless I shall follow your advice and post the log in a future message. Do you happen to know what is the shortcut for the error console on mac?

Here is the log:

pop3.server22.20: Got an error name=pop3StatFail, the server said: Server Unavailable. 21 3 Pop3Client.sys.mjs:1622:18

   _actionError resource:///modules/Pop3Client.sys.mjs:1622
   _actionStatResponse resource:///modules/Pop3Client.sys.mjs:1111
   _onData resource:///modules/Pop3Client.sys.mjs:352

Update. The problem disappeared this morning as suddenly as it appeared, without any action at my end.

I have the same problem just started recently after update to Thunderbird 140, but I'm getting the same message in Betterbird 128. The STAT command did not succeed. Error getting message number and sizes. Mail server outlook.office365.com responded: Server Unavailable. 21. I followed the advice to get the error event, but couldn't get the text, so copied the contents into an image.https://assets-prod.sumo.prod.webservices.mozgcp.net/media/uploads/images/2025-07-18-06-59-43-f7d239.png

Apparently this is a dysfunction of the server. It should fix itself within a few days, without any action at your end.