Usermin point to mail box

Hi all, This is my current layout.

Box 1

-Prof Edition -Running for Vhost web sites (domainA.com domainB.com etc)

Box 2

• GPL licence
• Holding mailBox (for domainA, domain B)

Reason separate box, is due to mail server required more cpu/bandwidth/diskspace and has list a big groups of users.

DNS

A/CNAME for websites, point to box 1 Respective MX Records point to box 2

I tested websites and smtp/pop run beautifully. The problem now is webmail - usermin.

Question

How to I configure http://webmail.domainA.com will point to usermin on Box 2 with URL unchanged? (which means I dont want to redirect the url) *current webmail.domainA.com will redirect to https://domainA.com:20000

Any idea, some said using port fwd in iptables. But i need hear what experts says ?

Any configuration on webmin can solve this issue?

Thanks, KH