Are all 5 servers in the same Routing Group? Are the 2
servers, out of the 3 other servers, that are not
receiving mail directly, eventually getting it from the
third? If mail is sent in the other direction, what
This would be easily explained by 2 routing groups, with
the first set of 2 servers set as local bridgeheads in
their routing group using the 1/3 server that is receiving
mail as the remote bridgehead.
If this is not the situation, I would look very carefully
at your smtp virtual servers and dns.