mail forwarding using ipfw

[free bsd]

dear all,
 i have network like this

lanX.com ---------
                  |
lanA.com ---- IPFW FBSD ---- lanB.com ===> to Internet
                |   |
lanC.com -------|   |------- lanD.com 

right now,... lanA.com , lanC.com, lanD.com have
smtpoutgoing to internet via lanB.com

the problem is ..  i want to make ruleset in IPFW FBSD
that email outgoing from lanC.com , lanD.com, and
lanA.com through lanX.com before go to lanB.com
without change configuration smtpoutgoing in the
lanA,lanC,lanD only in IPFW_FBSD .. (is it possible
???)


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