Quite simple:
If someone were to sniff the encrypted traffic between Hotmail and Gmail then they wouldn't have any idea who was talking to whom.
If someone sniffs the traffic between Hotmail and my server, it's trivial to see that a Hotmail user talked to me or one of the few others using my email server.
If someone sniffs the traffic between Hotmail and my server, it's trivial to see that a Hotmail user talked to me or one of the few others using my email server.