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.
Please Note: If using CCA to apply, re-apply, resety or midify your Cisco firewall, this entry may be re-inserted by CCA and you will have to manually remove.
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