After upgrading from exchange 2003 to exchange 2007 we have started receiving " 500 Firewall Error" in the mail logs for outside domain.
The issue appears when we send the bulk mails/multiple recipient with same subject and time stamp. The issue was completely random. sometimes mail delivered and sometime not.
We have cisco router and firewall at gateway. after some debugging we found that issue was due to IP INSPECT feature in cisco IOS.
After running the below command the issues disappears.
no ip inspect name SDM_LOW esmtp
ip inspect name esmtp
no ip inspect name esmtp