How to connect external account into Thunderbird

Hi there,

I have successfully connected an external mail account into my mailbox.org account, so I can now see the new account - including its inbox, sent folder, etc - in the web interface.

It is also fetching emails as expected.

The problem is that it is completely invisible to Thunderbird.

To be clear: I had already connected my main mailbox.org account with Thunderbird for a long time and it works without any issues - including all the aliases I have set up.

The problem is only related to the newly added external mail account.

I've already tried the "Subscribe" menu in TB, but it doesn't show up there either.

If anyone could tell me how to make it visible and usable in TB, I would really appreciate it.

Thank you!