ZyXEL Communications USG-100@USG-200 - V2.20 ED 2 - Page 431

Chapter 19 NAT
ZyWALL USG 100/200 Series User’s Guide
431
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 322 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