Messaging Gateway (SMG) cannot deliver messages to some domains. Message Audit Logs indicate that messages are rejected with a 550 reponse code by the destination mail server and the message is discarded from the delivery queue.
"550 5.5.0 invalid ehlo/helo host.example.com" in message audit log.
Some mail servers perform DNS checks to validate the connection including the following:
The connecing IP has a PTR record in the DNS
A lookup of the hostname in the PTR record returns the connecting IP (forward and reverse lookups match)
The hostname in the EHLO has a valid A record or MX record in the DNS
The domain part of the MAIL FROM has either an A record or MX record in the DNS
Symantec recommends that when using SMG for outbound delivery the SMG hostname, A records, and PTR records all provide consistant identification for the SMG system.
When sending and receiving messages this is the name that Symantec Brightmail Gateway will use to greet other MTAs.
To change the MTA host name:
Login to the Control Center
Click the Administration Tab
Navigate to -> Hosts -> Configuration
Click on the scanner(s) having the problem to open its configuration
Click the SMTP Tab -> Scroll down and click on Advanced Settings
Change the "MTA host name" to match the MX record found in the nslookup. (in this example it will be: "mail.example.com")
Once the changes are applied, retry the mail delivery:
Navigate to Status -> SMTP -> Message Queues
On the right-hand side select the Host you just changed, change the queue to "Delivery", change the list to "All"