The MTA’s LMTP server is more efficient for delivering to the back end message store because it:
Reduces the load on the back end stores.
Because relays are horizontally scalable and back end stores are not, it is good practice to push as much processing to the relays as possible
Reduces the load on the LDAP servers.
The LDAP infrastructure is often a limiting factor in large messaging deployments.
Reduces the number of message queues.
Having queues on both the relay and the back end store makes finding a lost message that much harder for the administrator of a messaging deployment.