Do you keep a lot of mail stored in Inbox?

Go to Account Settings > Server Settings, note the folder that appears in Local Directory, open that folder in the file manager of the OS you're using, and post a screenshot of the contents here.

Is setting up the account as IMAP an option, just to check whether Thunderbird can connect to the server and see the mail there using a different protocol?

As the requested the first level that opens as I folow the link from my account.

SIze of Inbox.sbd approx 300MB SIze of the inbox of the other acoount (on a different clinet 1,7 BG

There are only folders there, not files. You appear to be looking at the contents of the account folder from Thunderbird itself, probably by having clicked the Browse button to the right of Account Settings > Server Settings > Local Directory. I told you to note that directory, but display its contents using the file manager of the OS you're using, not Thunderbird itself.

Apologies for my misunderstanding. Here is an image of t mail directory

As of the second question; Is setting up the account as IMAP an option, just to check whether Thunderbird can connect to the server and see the mail there using a different protocol? If I change to IMAP, won't I loose my existing mail archives?

The contents of that folder don't show any signs of trouble there. What happens when you do manually check for new mail? Do you get an error message?

You cannot change the type of a mail account in Thunderbird. What I suggested is to set up the account as IMAP in addition to, not instead of, the POP account you have. The POP account in Thunderbird won't be affected by that other than if you remove messages from the server using the IMAP account, those messages will obviously not be available to the POP account for download.

The contents of that folder don't show any signs of trouble there. What happens when you do manually check for new mail? Do you get an error message? Answer. There are no error messages at all. Looked both at the client and server.

The two clients have two things in common. They are both in the same domain and they both stopped forwarding mail on Februari .18. I have three other accounts in other domains that operate as designed.

D you see a message on the status bar at the bottom of Thunderbird. something about getting message X of XX and then nothing appears?

The problem has been solved. It was surpisingly a problem with my provider me that Thunderbird resolves for a very long time. Probably the last update corrected somewhere an internal error made that my mail was wrongly received for a long time.

Appprox 5 years ago my provider was sold to another company and all mailsettings stayed the same for a long time. So my mail client retreived somehow his the mail at mail.xyz.com whereas it should have been mail.abc.com I just communicated with the provider and the mishapp is resolved.

It may newer be clear what triggered the change, but it now works as it should work.

Thnx for the help and apoligies for the time you invested.