Can that account be set up as IMAP? You could try to do that and see whether that sheds some light on what could be the problem…

Can that account be set up as IMAP?

in principle, yes. But I have 10+ years of email history locally in my thunderbird profile and only exactly 1 year on gmx.net (due to storage limitations) and I am afraid when I change that to IMAP, all emails older than 1 year will be gone...

I'm not asking you to change or remove your current POP account, but rather to set it up as IMAP while still keeping the POP setup you have (you may have both setups alongside each other), just so you may (1) check whether IMAP works and (2) be able to see what does Thunderbird see on the server.

And storing all that mail in your POP account Inbox, if that's what you're doing, is a bad idea anyway, could be related to the problem even. You may create folders under Local Folders, at the bottom of the list of accounts in the folder pane, to store and organise your mail locally on your computer there however you wish, independently of any account you may have.

[...] set it up as IMAP while still keeping the POP setup you have (you may have both setups alongside each other), just so you may (1) check whether IMAP works and (2) be able to see what does Thunderbird see on the server.

This does indeed work

And storing all that mail in your POP account Inbox, if that's what you're doing, is a bad idea anyway, could be related to the problem even. You may create folders under Local Folders, at the bottom of the list of accounts in the folder pane, to store and organise your mail locally on your computer there however you wish, independently of any account you may have.

Hm, interesting point. I do have some ~20k emails in that inbox, size is 2 GB in total of the Inbox folder, but I also have a bunch of folders under that account into which I sort emails (i.e. they are on the same level as the Inbox folder, and they of course only exist in thunderbird). Would that be any different than having the folders in "Local folders" ? And is 20k emails too much, and can I somehow fix the not-working connection of my pop3-account by "simply" cleaning up the "Inbox" folder?

Thanks!

Creating folders within the account makes more sense when it's an IMAP account, because in that case you may want your mail to be organised into folders at the server. But for mail stored locally, the use of Local Folders would be preferable, because mail accounts may come and go, you may want to have several of them, you may want to delete one that's having problems and set it up again, etc. Mail stored within Local Folders isn't affected by any of that.

I'm not aware of any hard limit on the size of a folder or the number of messages within it, but the more messages you store there and the bigger it becomes, the more likely it is that it may become corrupt and that you may eventually lose access to the mail stored there… to all the mail stored there…

This is because the contents of each folder are stored in a single text file with one message after another in mbox format. Things like software bugs, hardware failures, or malformed messages, may eventually cause Thunderbird to lose track of where a message ends and the next starts, and this is more likely to happen precisely with Inbox, because of the high number of message additions and deletions that happen there, so keeping that many messages stored in Inbox is definitely a really bad idea.

I don't know whether this could be the cause of the problem you have, I'd expect Thunderbird to report an error message if it was failing to add new messages to Inbox, but it's certainly a possibility to consider.

So yes, I would recommend you start by moving all the mail you currently have in Inbox into other folders, and then, when empty, right click on it and choose Compact. Don't compact the folder until all your mail has been moved out of there, because if there is some mbox file corruption, compacting the folder can only make things worse. Compacting is good maintenance practice when things work well to help prevent corruption, but shouldn't be used if there is some corruption already:

https://support.mozilla.org/kb/compacting-folders

I don't really expect this to fix the problem, but is something you better do anyway before trying other things.

Thanks for the long explanation! I deleted ~5k emails and now I am able to download mails again with the pop3 account. Seems like I should include more regular "cleaning-up" session in my schedule.

I also never used Local Folders so far, despite using Thunderbird for ... well, a very long time. One last question concerning "Local Folders": can I export and import the "Local Folders" only and ignore (i.e. not applying any changes to) all my other email accounts?

I deleted ~5k emails and now I am able to download mails again with the pop3 account.

Really? So that was the problem after all?

can I export and import the "Local Folders" only and ignore (i.e. not applying any changes to) all my other email accounts?

Not sure what you mean… Export how? You ask that as if it made a difference whether your mail is stored in Local Folders or in account folders for export purposes…

Thunderbird has limited built-in import/export capabilities, which are described here:

https://support.mozilla.org/kb/thunderbird-import https://support.mozilla.org/kb/thunderbird-export

In particular, and strangely enough, Thunderbird doesn't have built-in import/export capabilities for individual mailboxes, but that functionality can be added by installing an add-on called ImportExportTools:

https://addons.thunderbird.net/thunderbird/addon/importexporttools-ng/

The way Thunderbird manages mail on disk, however, makes it simple to import/export mail by merely moving files around. Thunderbird stores mail using text files in mbox format with no filename extension within the profile folder. Each mbox file corresponds to a mail folder in Thunderbird. The folder hierarchy shown by Thunderbird in the folder pane is derived directly from the way mbox files are organised in the filesystem within the profile folder.

You can import mail (i.e. make folders appear) in Thunderbird by merely placing the corresponding mbox file (only the mbox file, with no filename extension and no accompanying files) in the appropriate location within the profile folder. The next time Thunderbird is launched, it will automatically recognise the mbox file, create the associated .msf index file, and show it as a folder in the folder pane, with all the messages in it. And similarly, you can export mail by merely copying the corresponding mbox file to the desired location.

Not sure what you mean… Export how? You ask that as if it made a difference whether your mail is stored in Local Folders or in account folders for export purposes…

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Sorry, that was badly phrased: I was wondering if I can simply "copy" the "Local Folders" to another thunderbird installation (i.e. from my office desk-PC to my laptop), where slightly different mail-boxes are monitored. From the rest of your response, it sounds indeed like "Yes".

Ah, yeah. There is no downside to storing your mail in Local Folders when compared to (also local) POP account folders, only the advantage of that being a central place where all your mail can be managed independently of, and not being affected by anything done to, any of the mail accounts you may have.