sender_login_maps should read email.cc
When a mailbox forwards to other mailboxes via the cc ("send copy to") field, those destination accounts should be able to send mail as the original account, just like an alias/forward.
Use cases for not just using an alias/forward are when you want an autoresponder or filters for the mailbox.
It can use the 'disabledeliver' field to more or less emulate current behavior; if disabledeliver='y' (no local delivery, it's acting like an alias/forward), this new behavior will happen; if disabledeliver='n' (local delivery does happen), it's acting more like a standard mailbox and retains current/legacy behavior.