Request for Comments 5321

4.1.1.1.  Extended HELLO (EHLO) or HELLO (HELO)
    These commands are used to identify the SMTP client to the SMTP
    server.  The argument clause contains the fully-qualified domain name
    of the SMTP client, if one is available.  In situations in which the
    SMTP client system does not have a meaningful domain name (e.g., when
    its address is dynamically allocated and no reverse mapping record is
    available), the client SHOULD send an address literal (see
    Section 4.1.3).
    RFC 2821, and some earlier informal practices, encouraged following
    the literal by information that would help to identify the client
    system.  That convention was not widely supported, and many SMTP
    servers considered it an error.  In the interest of interoperability,
    it is probably wise for servers to be prepared for this string to
    occur, but SMTP clients SHOULD NOT send it.
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