3.2. Message Rejection and Bouncing MTAs and MSAs MAY implement message rejection rules that rely in part on whether the message is a submission or a relay.
For example, some sites might configure their MTA to reject all RCPT TOs for messages that do not reference local users, and configure their MSA to reject all message submissions that do not come from authorized users, based on IP address, or authenticated identity. NOTE: It is better to reject a message than to risk sending one that is damaged. This is especially true for problems that are correctable by the MUA, for example, an invalid 'From' field. If an MSA is not able to determine a return path to the submitting user, from a valid MAIL FROM, a valid source IP address, or based on authenticated identity, then the MSA SHOULD immediately reject the message. A message can be immediately rejected by returning a 550 code to the MAIL FROM command. Note that a null return path, that is, MAIL FROM:<>, is permitted and MUST be accepted. (MUAs need to generate null return-path messages for a variety of reasons, including disposition notifications.) Except in the case where the MSA is unable to determine a valid return path for the message being submitted, text in this specification which instructs an MSA to issue a rejection code MAY be complied with by accepting the message and subsequently generating a bounce message. (That is, if the MSA is going to reject a message for any reason except being unable to determine a return path, it can optionally do an immediate rejection or accept the message and then mail a bounce.) NOTE: In the normal case of message submission, immediately rejecting the message is preferred, as it gives the user and MUA direct feedback. To properly handle delayed bounces the client MUA must maintain a queue of messages it has submitted, and match bounces to them. http://www.faqs.org/rfcs/rfc2476.html - To unsubscribe from this list: send the line "unsubscribe linux-newbie" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.linux-learn.org/faqs
