Reject Ndr

• The mail exchanger should reject the transfer of a message outright if it can't be delivered, but some MXs send this job to the MDA.
• Since the MDA can't "reject" the mail since it's already been accepted by the MX, the best it can do is to send a bounce message back to the return-path address indicating that the mail is undeliverable.
• Graylisting is a technique used by a mail exchanger host to defend against spam deliveries by temporarily rejecting all mail from unrecognized sources.
• A good spam filter software program will allow you to set up features regarding desired email, including which messages to accept and reject (white and black lists), as well as which to organize into folders.
• Blacklists are a one form of spam filter; they usually do not block or reject e-mail so much as merely route it to some destination of the user's choice (like the junk mail folder).
• Blacklists are not a terribly effective form of spam filter as they usually do not block or reject e-mail as much as merely route it to some destination of the user's choice (like the trash can).
• Graylisting requires a bit of extra work by the mx (particularly for large mailing services that must coordinate among many mx hosts), but possibly not as much cpu time as would be required to process and filter the spam if it were not rejected.
• For example, if you define cialis as a spam word, then every e-mail that has the word cialis in it will be rejected or placed in your junk mail folder.
• So, if the mail provider has set up blocklisting or other facilities to reject spam deliveries, then it is spared the necessity of filtering most such mail for its users.
• Probe messages are evidence of a dictionary attack (or "directory harvest attack" or "mx probe") in which the spammer sends probe messages to a large number of e-mail addresses (some of which may be guesses) and prunes out those that are rejected by the mx.
• If a sender's ip appears in a blocklist, the mail host can reject delivery of the message so that it will not show up in the recipient's inbox.

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