Sender-dependent Transport Maps: Change Sender Address

thn80

I am using a Sender-dependent Transport Map to forward all mails from one user to the GMX SMTP-Server. This user has a GMX account and the mails shall be sent into the Internet via this account.
However, when mailcow forwards a mail to the GMX server, the server response is “smtp; 550-Requested action not taken: mailbox unavailable 550 Sender address is not allowed”.

Is there a way to force the sender address of such mails from the user’s mailcow address to the user’s GMX mail address, so that the server accepts those mails?

Thanks a lot in advance.