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.
- 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.