How to configure 4 mailservers to work with 4 Virtual IP's?

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Hi guys

I have 4 mail servers on my network.  I have 4 VIP's (Virtual IP) and one RIP (Real IP).

Without using 1:1 NAT - How do I associate outbound traffic from mailserver A to VIP 1, and mailserver B to VIP 2…

Presently all outbound mail looks to originate from the RIP.  This makes all our mail look like spam since the originating addy doesn't match up with where it says it should be coming from.

Thanks in advance!

(Apologies to Mods - I posted this question in wrong place previously and am reproducing it here)

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er, nobody knows or nobody cares?

[dotdash]

There's another thread dealing with a similar problem that might help:
http://forum.pfsense.org/index.php/topic,5213.msg31442.html#msg31442

[cmb]

Use Outbound NAT.