Where i work, we have an email server in r network, and had that same issue till we REMOVED the router and decided to just use ZS. so r config, and has worked ever since, is: modem ZS network, where network is 192.168.0.0. We used to have modem router ZS network and even w/ all ports forwarded wouldnt receive emails.
imo drop the router, ZS is head and shoulders above any router device, dlink/linksys/etc…
just remember to set, under “network/router/virtual server”, ALL the ports u need. w/ the apps we use there r approx 9 rules. having a static ip for mail server will help greatly, use dhcp ip/mac.
hope this helps u. took me a week to convince my boss at the time to do just that.