Suddenly we've started to receive the following message:
OES11sp2, GW2012

In Email

Transcript of session follows:
Command: gmail.com
Response: 550 Host unknown (gmail.com)

In GW Logs:

10:20:37 F487 MSG 3314268 Detected error on SMTP command
10:20:37 F487 MSG 3314268 Command: gmail.com
10:20:37 F487 MSG 3314268 Response: 550 Host unknown (gmail.com)

It is only happening to mail sent to gmail accounts and is very, intermitent. I have not been able to recreate the probelm no matter what I do, but I do see the errors in the logs, It is so intermitent that if I ask the person to resend the email most of the time it goes through without a problem.

We did change external IP a few weeks ago, but with our setup, I only need to change the Firewall settings. I've gone through everything config I can think of on the FW and in GW looking for extra spaces, commas, periods, etc. I've even gone as far as putting google's DNS at the top of the list figuring worse case would be that all gmail would go through but something else would fail, but nope, still very intermittent 550 Unknown host (gmail.com) errors.

If anyone has any ideas, I would appreciate it.