This primarily allows an email mailbox to disable local delivery and become forward-only, but still allow smtp auth for sending (which a Mail Forward does not). Fixes #5011 (closed)
Also respects the outgoing bcc setting of a mail_user when sending from a mail alias or mail forward.
Also fixes outgoing bcc to only allow a single email address. Fixes #5298 (closed)
Fixes #2387 (closed)