Ok, I figured out the path thanks to the link below:

Now I dont get a mailer daemon email with an error (not in the recipient inbox nor in the sender inbox – I really got the two cases) and when writting from the FEP, there is no problem. The recipient gets the message and the email… BUT, if the recipient replies the email (yes, with a key in the subject line), there is not a registration in the FEP. I thought that was the whole point of email piping.

Am I getting it wrong?