Everything else seems to be fine. Keep in mind that as you add the number of nodes, you add to the likelihood that a server will have to lookup and forward the email to another server.
You can cheat on IMail's MX for peering. If you have a significant differences in mail traffic into each peer, then rather than have all the MX's at same preference, you could put the highest traffic server(s) as primary MX, and the others as backup. This will force all mail to the primary(s), where most of it will be delivered (the 80/20 rule would be welcome hee), and then the primary(s) relay the crumbs to the other site(s).
This can add traffic to your network, although I don't think anyone's quantified exactly how much (Len?).
As long as the traffic is trivial, peering should be fine.
I'm very interested in the answers to my questions to the 6-peer / 6-site user.
IMGate as MX and mail routing hub would be much more efficient and scaleable, since all mail would be delivered precisely (no VRFY probing to see who's got the mailbox). A question is whether "send all mail through (IMG)ateway" delivery overrides peering delivery direct to other peers.
Len
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