With Exchange 2007 both MAPI and SMTP connections are both catagorised under the same. On a hub transport server you can set the amount of outbound connections it can create.
This exchange 2007 hub transport server I set a maximum of outbound connections to be 10. However it can send up to 5 emails at once per domain.
This company they have a bad link, and they have lots of large attachments sitting in their queue up to 10MB in size. They recieve alot of 421 4.4.2 Connection Dropped due to packet loss etc. As you see below their MAPIDelivery is building up.
This is because it can only send 10 emails at once max, with 5 per domain. It needs to get through the queue to get back to MAPI. When it does, it goes and sends them basically instant to the mailbox server (because its over LAN high speed network) then goes back through the queue again.
However if you had an edge transport server you would not have this problem! Edge will just be sending SMTP traffic. It's the hub transport servers that will send both SMTP and MAPI.