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Nolisting is a method of defending electronic mail users against e-mail spam. The idea is that by having a ''non-existent'' primary mail server and a working secondary mail server, attempts to contact the primary mail server will always fail. If the mail is sent by a correctly configured email server, the sending server will then try to contact the secondary mail server, and should succeed. Spammers frequently use custom software that does not retry low-priority MX records. Nolisting refers ''only'' to a configuration involving an unresponsive ''single'' primary MX with one or more functional lower priority MX servers. This configuration has been proven to be very safe. Variations involving multiple unresponsive MX records do not meet this definition (and there is little evidence supporting the effectiveness of such an approach). Spammers are known to sometimes bypass the primary mail server and contact the lowest-priority (highest preference number) mail server first (in violation of RFC 2821). This is because backup mail servers often have less stringent spam filters and security checks and spam that would get blocked by the primary is often accepted by the secondary. This is an old technique that Nolisting does not specifically address; however, it does increase the likelihood that the spam will be handled by the working secondary mail server (especially in environments with no backup MX). Unlike real email servers, spam sources often don't retry on failure. Thus the failure to deliver on the first attempt causes the spammer to move on to the next victim. Legitimate email servers will retry the next higher numbered MX and the email is delivered with no significant delay. The result is that a significant amount of spam bot spam just goes away. It also reduces the load levels on the server as less spam has to be processed by spam filtering software. This technique is sometimes referred to as "poor man's greylisting" because it is easy and inexpensive to implement. ==Drawbacks== Downsides to this technique include increased traffic from those spam programs that send to all MX records listed, and the danger of unknowingly losing mail from an improperly configured mail transfer agent (MTA) or script. At least some (nolisting uncovers applications written using JavaMail do not retry ) using the next highest priority MX. This technique relies on spammers using custom software that ignores the SMTP protocol. As such, it is not a viable long-term solution. Spammers can thwart Nolisting by simply using standard email server software or by adding a little error-recovery to their custom software. Thankfully, Nolisting can be easily abandoned if it ceases to be useful. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Nolisting」の詳細全文を読む スポンサード リンク
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