This article contains additional information on the requirements to enable SMTP Authentication on a Symantec Messaging Gateway.

As mentioned in the Messaging Gateway's Administration Guide, a dedicated socket is required to enable SMTP Authentication for outbound email communications.
This is to ensure higher security on authtenticated communications, as also recommended by RFC 4954.

For more information on SMTP authentication, see RFC 4954: http://www.ietf.org/rfc/rfc4954.txt