Symantec Messaging Gateway (SMG) does not accept messages when envelope sender or recipient email address(es) contain underscore character in the domain name.

501 5.5.2 MAIL FROM syntax error

501 5.5.2 RCPT TO syntax error

This behavior is by design.

The product is behaving in according with RFC standards.  Specifically, RFC5321 does NOT permit use of underscore character in domain names.  According to RFC5321:

QUOTE:
4.1.2 Command Argument Syntax
...
   To promote interoperability and consistent with long-standing
   guidance about conservative use of the DNS in naming and applications
   (e.g., see Section 2.3.1 of the base DNS document, RFC 1035 [2]),
   characters outside the set of alphabetic characters, digits, and
   hyphen MUST NOT appear in domain name labels for SMTP clients or
   servers.  In particular, the underscore character is not permitted.
   SMTP servers that receive a command in which invalid character codes
   have been employed, and for which there are no other reasons for
   rejection, MUST reject that command with a 501 response
UNQUOTE:

Please make sure that the MTA, that is trying to deliver mail to SMG, is using RFC compliant SMTP command syntax and parameters.

References:

http://tools.ietf.org/html/rfc5321