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I'm getting lots of junk "return to delivery" mail on my server, so I would like to add this "RDNS" lookup thing in Ubuntu.

But I can't figure out how. Most documentation is about regular DNS servers.

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You do not specify what software your email server is? Exim? Sendmail? Postfix? – adamo Mar 4 '11 at 12:06

Assuming from the tagging of the question that your mail server software is Postfix, you can tune this via the smtpd_client_restrictions variable as described in postconf(5)

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