you need src NAT:
/ip firewall nat
add chain=srcnat action=src-nat to-addresses=currect_Public_IP src-address=IP_of_email_server
place this above you masquerade rule in NAT
Hi Solar, I am still reading the other post, but I see a similar item that is nagging me and specifically the use of to-addresses
I am not in front of a hex at the moment and thus trying to imagine making the rule.
I understand source and destination and Interfaces in out, but where the heck is TO??????? (purpose, use and differentiation from the other typical directional items)?
My simple, none technical understanding, is that the action src-nat, change the source IP address, you can specify exactly what the IP is by using to-address.
if you use action=masquerade, it means change source IP to the IP assigned to the out-interface. which is what you normally need for NAT your internet access through WAN interface in a standard single WAN network.
Dst-nat, is to change the destination IP address, again, you specify what you want to change it to, by using to-address. typical use of this is port forwarding, so traffic heading to your WAN IP address, is then changed to your LAN IP so it knows where to go.
Hope this helps.