Message delivery fails or is slow for some domains when you have Cisco hardware in your network environment. This results in messages that are held in the Symantec Messaging Gateway (SMG) delivery queue, with the following SMTP codes:
451 4.4.2 [internal] connection closed by remote host
451 4.4.2 [internal] no HELO/EHLO response
This behavior is commonly caused by SMTP filtering by a Cisco firewall or router. Depending on the Cisco hardware installed, this may be called SMTP Protocol Inspection, SMTP Fixup, or Mailguard.
Ensure that the packet shaping or filtering that is caused by the Cisco hardware is disabled for SMTP traffic.
Search Cisco Support for documentation on how to change this feature.