= SMTP/RFC5321 =

について、ここに記述してください。


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

7.9

In recent years, use of the relay function through arbitrary sites
   has been used as part of hostile efforts to hide the actual origins
   of mail.  Some sites have decided to limit the use of the relay
   function to known or identifiable sources, and implementations SHOULD
   provide the capability to perform this type of filtering. 

古そうな内容だが。

 When mail
   is rejected for these or other policy reasons, a 550 code SHOULD be
   used in response to EHLO (or HELO), MAIL, or RCPT as appropriate.