If I create an account without a local mailbox (forward only), I'm unable to send email from that account. I get the following error when trying:

SMTP Error (553): Failed to add recipient "bar@example.com" (5.7.1 <foo@example.com>: Sender address rejected: not owned by user foo@example.com).

This seems wrong as I'm authenticated as the sender address so it should be considered valid. I'd like to setup some no-reply accounts for services I have running and this is preventing it.