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:
Jan 31 01:04:21 mailserver postfix/smtp: 34CA233700CB: host mx.east.cox.net[126.96.36.199] refused to talk to me: 421 eastrmimpi03.cox.net IMP 188.8.131.52 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: 94FC33370078: to=<ShanellMedsker1023@interbgc.com>, relay=mail.interbgc.com[184.108.40.206]:25, delay=221697, delays=221693/0.28/3.5/0.62, dsn=4.7.1, status=deferred (host mail.interbgc.com[220.127.116.11] 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=18.104.22.168&receiver=p1.interbgc.com (in reply to RCPT TO command))
Jan 31 01:04:26 mailserver postfix/smtp: 94F303370060: host mailin15.datevnet.de[22.214.171.124] said: 450 4.7.1 Client host rejected: cannot find your reverse hostname, [126.96.36.199] (in reply to RCPT TO command)
Jan 31 01:04:30 mailserver postfix/smtp: 95B0F33700DB: to=<email@example.com>, relay=smtp-in2.nuvox.net[188.8.131.52]:25, delay=67120, delays=67108/0.27/12/0.05, dsn=4.7.1, status=deferred (host smtp-in2.nuvox.net[184.108.40.206] said: 450 4.7.1 Access temporarily denied. Cannot resolve PTR record for 220.127.116.11 SRC001 Contact firstname.lastname@example.org (in reply to MAIL FROM command))
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