Watchguard being selective on what domains it e-mails to

[superjet]

OK.
So about a month and a half ago, I got a Firebox X. I configured it and it worked fine for about three weeks. Then it decided to block e-mails to some domains.
At first, I thought there is no way it is the firewall, but I can't telnet to the receivers e-mail server from inside the network, but I can from my router on the other side of the firewall.
I put our old Firebox II back in place (after I reconfigured it completely) and it worked fine for about three weeks.
Now there are e-mails held up in the Exchange outbound Queue with "Destination host unreachable"
Does anyone have any idea what is causing this?
Thanks,
Jamie

 

[oceanrover]

Is this a reverse lookup issue? Is the External of the Firebox your MX record? If it is not, is One-to-One NAT setup so that Outgoing mail actually comes from the registered IP? Just some ideas.

 

[superjet]

Thanks for the reply.
Reverse lookup works just fine. Checked with

http://www.dnsstuff.com

External of the firebox is not the MX record, but I do have one to one nat set up.
Should I have one to one NAT set up for outgoing as well as incoming?