the accounts had been migrated to the new servers, we noticed that we were seeing a lot of duplication in emails & active SMTP delivery threads since messages were being duplicated roughly half of the time.
Duped messages is a not feature of peering. What was causing your dupes?
This is compounded if your IMail servers forward all email through anti-virus / spam gateways (as the email could potentially hit the gateway twice)
So Imail SMTP "send all outbound through gateway" routing does override peering routing?
Peering will reduce the burden on a server for IMAP/POP/auth requests
Well, yes, less mailboxes, less mailbox reading, but peering increases the "burden" on mailbox readers since each mailbox reader must connect to the specific peer holding his mailbox.
but may not necessarily reduce overall SMTP traffic
huh? peering will necessarily ALWAYS increase SMTP traffic in EVERY peer (ie, peering is not a scaling solution, because it is not a scaleable solution). It's a convenient routing solution if your traffic is sufficiently small.
Here's my so-called "kneejerk" analysis:
Inbound msgs will hit the right peer (out of x total peers) only 1/x of the time, and so (1 - 1/x) of the inbound at will arrive at the wrong peer and wil need re-delivery to correct peer.
Let's say x = 6 peers. 1/6 of the MX traffic will arrive at a peer, but only 1/6 of that will be for that peer's mailboxes. 1/6 * 1/6 => 2.7%, or 97% of the traffic goes to the wrong box. oops!
ie, for 100 messages into a peer, only 3 will be for that peer's mailboxes, and the peer has to VRFY probe the other 5 peers to deliver the other 97 messages.
100 msgs/hour? No big deal. But 10,000 msgs/hour? That's 9700 msgs/hour that must re-delivered to the right peer, plus ( 2.5 x 9700 ) VRFY probes to find the right peer. "Houston, ..."
Ah, but you say "we have only 2 peers"? Great, you have the "best case" peering, so "only" 50% of the messages will have to re-delivered. A peer receives 5k msgs/hour, and 2.5K must be re-delivered to the other peer, doubling my peer's traffic, and that's best case.
Except for the tiniest quantities of inbound traffic, it's better to forget about peering, and run one mailbox server.
Len
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