From Computer Tyme Support Wiki

Preventing your email server from being blacklisted

Most spam filtering companies do the best they can to pass good email. Often there are problems where good email gets blocked. One of the factors that contributes to good email getting blocked are email servers that aren't properly configured. Doing it right makes a big difference and many easy steps can keep you from getting blacklisted. And it makes your server a candidate for white listing which will get you through some spam filters faster. We at Junk Email Filter encourage you to follow these guidelines to help us and our competitors deliver your good email.

Getting your Reverse DNS correct

One of the biggest things you can do is you get your reverse DNS correct. And to really do it right you need to have Forward Confirmed Reverse DNS set correctly. This is a very big step towards getting your email delivered correctly so it's worth putting out the effort to get it right.

Reverse DNS (RDNS) is a host name that is returned when looking up an IP address. For example, lets say that your domain is called mydomain.com and your IP address is 1.2.3.4. The first step is to set a PTR record for 1.2.3.4 that returns mail.mydomain.com. Often you won't have control over this directly but your hosting provider does. Ask them to set your RDNS for your IP address.

But setting the RNDS for your IP is just half of the job. The RDNS returns a host name for your IP address. But to do it right that host name that is returned has to point back to the original IP. This is what is called Forward Confirmed RDNS or FcRDNS. The host name is an A record and more likely under your control.

1.2.3.4 -> mail.mydomain.com - PTR Record
mail.mydomain.com -> 1.2.3.4 - A record