Looking at your diagram the mail server is connected to network A and network D. I suspect the default-gateway on the mail server is pointing to an address on network D.

Jack works because he is the same network as one of the nic cards on the mail server so the default-gateway is not relevant.

John doesn't because when the mail server needs to send traffic back to John it uses it's default-gateway and sends it out to D.

So you have 2 choices -

1) On the Cisco 2811 NAT John's mail traffic to the 192.168.10.2 address. That way the mail server will be able to send the traffic back to John

OR

2) on the mail server itself add another route for the 192.168.40.0/24 network so the mail server knows how to return the traffic -

route -p add 192.168.40.0 mask 255.255.255.0 192.168.10.2

you need the -p in the above command to make sure the route stays across reboots of the mail server.

Jon