Sam,
Tracking emails would be a whole other world of hurt.
For PMs, there are other platforms which distinguish between “Outbox” and “Sent”, but this is a bit propellorheadish for many users.
Multi recipient PMs: How about a symbol placed next to the time, when the user clicks on it it shows a list of who the PM was sent to, with “Read” or “Unread” next to each username, added bonus points if it also indicates “Mail sent” for a user who has email list mode, or notifications enabled.
(I am aware that earning the bonus points won’t be particularly easy).
If it can’t be done in a way which doesn’t preclude partially correct information, then it isn’t worth doing.