Is Gmail the account from which the messages are forwarded to the GMX account?

Is Gmail the account from which the messages are forwarded to the GMX account?

No.

Edit: If they were, why would it matter? I thought its up to where the emails are located.

So how is Gmail involved in this? Is it one of the accounts you have in Thunderbird besides GMX?

Yes, both are setup in Thunderbird.

And the messages get uploaded from one account to another, which shouldnt happen i would think.

Check the Account Settings > Copies & Folders settings for both accounts and confirm that archived messages are configured to be saved in each account's respective archive folder. For Gmail, the IMAP archive folder is called All Mail. Use the "other" option then point it to the correct archive folder.

Also, is the GMX account the one receiving the forwarded messages, yet its not the To address? How then are those messages getting delivered to the GMX account if its not the recipient? Do you mean Cc'd or Bcc'd and not forwarded?

What have the settings of the Gmail account to do with that?

to your second question: yes, the emails are forwarded by an anti-spam provider. Therefore i also cant set up an alias in the options, because there are infinete possiblities.

What have the settings of the Gmail account to do with that?

It's part of your setup and we're not leaving any stones unturned. Does this make you uncomfortable?

Well when my emails get moved from account B to account A, i cant see how the options in account A are relevant.

I think its a bug, but i dont have an account on bugzilla.

You can always register for a bugzilla account.

I am not a developer, i dont think they want non experts there.

So you dont have any idea why this problems occurs, (apart from wrong options)?

Ordinary users are filing bugs after extensive investigation is done here in the forums, but since you seem pretty sure it's a bug and not misconfiguration, then there is nothing else I can tell you other than to file a bug report yourself, because only you know why you think it's a bug. If this were a bug, we'd have seen a lot of similar reports here in the forums and at least one bug report would have been filed by now if that were the case, but I haven't seen any other report of the same malfunction, which means this is unique to your setup and hence your configuration needs to be looked at.

No, I can't think of anything else other than wrong configuration.