If you are on a 100.0.0.0/24 network, which is a public network, you usually will never be able to get to a 192.168.1.0/24 network, because 192.168.1.0/24 traffic will never be routed on a public internet.
Or do you mean they have a 10.0.0.0/24 network?
In that case you need to tell the pf.sense box that 10.0.0.111 is the gateway for the 192.168.1.0/24 network.
Code:
route add -net 192.168.1.0/24 10.0.0.111
This is one part of the deal
It will now route packets for 192.168.1.20 through 10.0.0.111.
The second part is to get the answer packets.
To reply you, the mailserver needs to know that it should route 10.0.0.0/24 packets through 10.0.0.111.
But doesn't defeat all this the separation of the mailserver from the 10.0.0.0 net into it's own network?
J65nko - who has never use pfsense