Basically you should always be 100% sure that whatever IPyour 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=
Jan 31 01:04:26 mailserver postfix/smtp: 94F303370060: host mailin15.datevnet.de[184.108.40.206] said: 450 4.7.1 Client host rejected: cannot find your reverse hostname, [220.127.116.11] (in reply to RCPT TO command)
Jan 31 01:04:30 mailserver postfix/smtp: 95B0F33700DB: to=
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