Consequently, this delay can discourage spammers from using your server to send spam. However, keep in mind that the RBL also includes server addresses that are the victims of unauthorized relaying, so you may end up tarpitting servers that send legitimate mail. The RBL is an effective way to reduce the spam you receive. It is important to keep in mind, however, that if you enable the RBL test, you may inadvertently refuse valid email from legitimate sites that may be unaware that they have been the victims of unauthorized relaying and are blacklisted. All mail servers must have a reverse DNS host entry.
Spammers typically violate Internet mail standards.
If you configure your mail server or anti-spam software to reject mail that does not comply with Internet standards, you can eliminate a great deal of spam. Keep in mind, however, that blocking mail from servers that do not allow reverse DNS look-up may also result in blocking mail from legitimate sites that have improperly configured servers. This message is particularly helpful for customers of ISPs, who can then inform their providers of the problem and encourage their ISPs to configure their mail servers appropriately and allow delivery of mail to servers that reject mail that does not comply with Internet standards. Although not explicitly stated, valid host values for the HELO command are also encouraged by the Internet standards. Since every mail server on the Internet should have reverse DNS lookups defined, every mail server should also provide a valid hostname as a HELO value.