Using Posteo w/a Custom Domain?

[Kodachrome]

If you have your custom domain hosted with addy, you can send mail outbound through addy from any of the delivery addresses that you have configured with addy. I do this all the time. You can choose which alias addy will use as the sender and can even encrypt the mail with PGP between the actual sender and addy.

Or am I misunderstanding the question?