[求助][Juniper SSG5]Outgoing using real IP address other than interface IP

Hi all,
Real IP Mapping example as below

Real IP => Internal IP
113.28.100.1=>192.168.0.1
113.28.100.2=>192.168.0.2
113.28.100.3=>192.168.0.3
113.28.100.4=>192.168.0.4 (Email Server)
113.28.100.5 (untrust interface IP on router)

我地有4粒real IP同一個interface (route mode) 入,咁喺interface度設定左 Mapped IP 去返唔同對應既 internal IP度(Pls. see above),其中一粒係去我地既email server,咁我希望可以做到所有email server經router出街既smtp package用返 113.28.100.4 果粒IP 出街,我試過去設定一條policy 做source translation但佢要求我先setup DIP,但我喺untrust interface裡面既DIP唔可以setup (113.28.100.4) 話 ##DIP parameter error,請問有無高手可以講下可以點做到我需要既效果?謝謝。

補充:

而家所有package出街都會用左 Router untrust果粒 IP 即係133.28.100.5 出街。