Relay Mail to external SMTP server

I recently switched to Mailbox.org from a self hosted mailcow. I was able to migrate almost everything. I still have one problem: I have a couple of mail accounts that I used to centralize in one account. I managed easily to get all mails in the mailbox.org account. Unfortunately one of the domains has a DMARC policy of reject. So although I configured the Alternative Sender my mails are rejected. This is as expected.

In my self hosted solution I just configured a relay to the original SMTP server. So the recepient mail server was happy. As I already added my credentials to retrieve the mails from the external mailservers, could you add the feature to also allow to relay the mails to the external servers instead of sending the mails directly?