Basically you should always be 100% sure that whatever IP your mail server sends out with has reverse DNS/PTR records.  Remember that unless you own your IPs then you won't be able to set your own reverse DNS.  Even if you were to create a reverse PTR record on your DNS servers it will be ignored. Reverse DNS is queried to pre-assigned DNS servers of your ISP, so therefore you'll need to contact your ISP/Colo/Hosting provider to do a reverse DNS entry.

If you don't have reverse DNS you'll find many of your outgoing e-mails will be rejected and won't reach the recipient. You can usually check /var/log/maillog for lines like this:

• DNS check failure
• Client host rejected: cannot find your reverse hostname
• Cannot resolve PTR record

Jan 31 01:04:21 mailserver postfix/smtp[20217]: 34CA233700CB: host mx.east.cox.net[68.1.17.3] refused to talk to me: 421 eastrmimpi03.cox.net IMP 12.55.42.121 DNS check failure - try again later - Refer to Error Codes section at http://postmaster.cox.net/confluence/display/postmaster/Error+Codes for more information.

Jan 31 01:04:22 mailserver postfix/smtp[20334]: 94FC33370078: to=<ShanellMedsker1023@interbgc.com>, relay=mail.interbgc.com[217.9.224.121]:25, delay=221697, delays=221693/0.28/3.5/0.62, dsn=4.7.1, status=deferred (host mail.interbgc.com[217.9.224.121] said: 450 4.7.1 <ShanellMr1023@interbgc.com>: Recipient address rejected: temporary failure: Please see http://spf.pobox.com/why.html?sender=postmaster%40mail.mailserver.com&ip=12.55.42.121&receiver=p1.interbgc.com (in reply to RCPT TO command))

Jan 31 01:04:26 mailserver postfix/smtp[20337]: 94F303370060: host mailin15.datevnet.de[193.27.50.115] said: 450 4.7.1 Client host rejected: cannot find your reverse hostname, [12.55.42.121] (in reply to RCPT TO command)

Jan 31 01:04:30 mailserver postfix/smtp[20332]: 95B0F33700DB: to=<lamo2825@nuvox.net>, relay=smtp-in2.nuvox.net[70.43.63.21]:25, delay=67120, delays=67108/0.27/12/0.05, dsn=4.7.1, status=deferred (host smtp-in2.nuvox.net[70.43.63.21] said: 450 4.7.1 Access temporarily denied. Cannot resolve PTR record for 12.55.42.121 SRC001 Contact postmaster@nuvox.net (in reply to MAIL FROM command))

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