5.1.0 500-'Firewall Error'

When trying to send external emails out we were getting the following error randomly for particular emails:

[203.10.1.143] #<[203.10.1.143] #5.0.0 smtp; 5.1.0 - Unknown address error 500-'Firewall Error' (delivery attempts: 0)> #SMTP#;

CAUSE: The Cisco firewall has a configuration entry like the following (it may have additional parameters specified after in addition to esmtp):

ip inspect name esmtp

This problem occurs because of incompatibilities or restrictions caused by the Cisco firewall configuration. It is more likely to occur if you are sending an email to multiple recipients or using a distribution list in Exchange.

FIX: Disable this entry in the Cisco firewall configuration by inserting the word “no” at the beginning of the line as shown, so that it should now read something like

no ip inspect name esmtp

Other people have seen this problem as well:
http://www.solutions.pro/email-server-generates-500-firewall-error-error-8/
http://nzschooltech.blogspot.com/2009/09/exchange-management-console-issue-on.html
http://mattlog.net/2008/12/31/exchange-2007-500-firewall-error/