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Reverse-lookup failures

Mail from sites that fail a reverse-lookup test -- that is, when the name of the site is checked for its numeric IP address, and either

1. one cannot be found, or
2. the one found doesn't match the one from where the message actually came.

There are normally two possible causes for this:

1. Faking a site name -- this is one of the most common ways of concealing the source of e-mail, so it isn't surprising that this restriction has caught a large number of spam attempts, with addresses such as 64236028@13963.com, money@guaranteed.com, and thetruth@out.there.
2. ISP sloppiness/laziness -- A quite legitimate reason for this problem is the mail sender's Internet service provider's failure to assign names to all the numeric IP addresses it lets its customers use. Anyone having mail bounced for this reason should check with his or her ISP about this possibility.