Help needed with Watchguard Firebox III config

I have (2) SMTP servers behind a Watchguard III Firewall. The SMTP servers are on different domains.

SMTP-1 has a private address of 192.168.1.100 and a public address of

I configured the Watchguard for 1-to-1 NAT.

I am able to send and receive mail from both servers with the following excption: Email's sent from SMTP-2 to any address on SMTP-1 are queued and never delivered.

I am able to ping 192.168.1.100 (SMTP-1) from 192.168.1.95 (SMTP-2). I am unable to ping 207.x.x.148 (SMTP-1) from 192.168.1.95 (SMTP-2)

Can someone point out what I'm missing?

Thanks