ZyXEL Communications 1000 Manuel D’Utilisation

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 Chapter 19 NAT
ZyWALL USG 1000 User’s Guide
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SMTP server replied directly to the LAN user without the traffic going through NAT, 
the source would not match the original destination address which would cause 
the LAN user’s computer to shut down the session.  
Figure 295   LAN to LAN Return Traffic    
192.168.1.21
LAN
192.168.1.89
Source 1.1.1.1
SMTP
NAT
Source 192.168.1.21
SMTP