Followed the instruction below, it fixed bounce back mail issue. Why cisco firewall cause this problem?
Diagnostic information for administrators:
Generating server: <local Exchange server FQDN>
<remote SMTP server FQDN> #500 Firewall Error ##
CAUSE: The Cisco firewall has a configuration entry like the following (it may have additional parameters specified after <inspection-list-name> in addition to esmtp):
ip inspect name <inspection-list-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 <inspection-list-name> esmtp