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 often 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.
Alternately, the remote host may be closing the SMTP connection based on the receiver's IP block list or reputation service.
If using CISCO firewalls or packet inspection
If no firewall or content inspection is configured for SMTP traffic, the receiving domain may not be accepting connections from your domain or IP.