Show source IP behind NAT

maruscya Posts: 1
Friend Collector
edited April 2021 in Security
Dear Friends, 

I'm a newbie user of ZyXel USG-200. 

I need to reach SMTPs service behind the firewal. Following some guide i setup correctly the NAT service. 
Now the SMTPs is are available. 

I check the log on SMTPs server and all incoming connections comes from the IP of the firewall 

postfix/smtpd[8745]: DDEF012004D: client=unknown[
postfix/cleanup[8746]: DDEF012004D: message-id=<>
postfix/qmgr[381]: DDEF012004D: from=<*************>, size=3762, nrcpt=1 (queue active)

In this example the IP of google is replaced by the IP of the LAN of firewall (
I try to find how to resolve this issue by myself but without success. 

WAN2_IP : The public IP of firewall
LAN IP : of firewall 
MailCow: IP of mailserver 

Can someone help me? 

Many thanks


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