RFC 2821

RFC2821, section 4.1.2: Command Argument Syntax:

(near the end)

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, RFC1035 [22]),

characters outside the set of alphas, 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.