The Exchange transport engine is pretty solid and its basics did not need much improvement in Exchange 2010 SP1. Microsoft therefore concentrated on fine-tuning transport in three areas:
SMTP load-balancing and failover: Making sure that Exchange balances connections most efficiently when an SMTP server fails.
Submission queue resource monitoring: Making sure that a server that is under stress is not placed under further stress by having to deal with new message submissions. This is a variation of the pressure backoff work that was first introduced in Exchange 2007.
Mailbox delivery prioritization: Making sure that the general flow of messages isn’t slowed down by the email equivalent of slow-moving trucks in the outside ...