ZyXEL Communications ZyWall USG 300 User Manual page 421

Chapter 19 NAT
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 319 LAN to LAN Return Traffic
NAT
Source 192.168.1.21
Source 1.1.1.1
SMTP
SMTP
LAN
192.168.1.21 192.168.1.89
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ZyWALL USG 300 User's Guide