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

Once your FcRDNS is correct then you can be white listed by host name in addition to by IP address. So spam filters block IPs with no RDNS and some even block you if FcRNDS isn't correct. But even if you aren't blocked then bad or missing RDNS counts against you and makes it more likely that your email will me mistakenly listed as spam.

Setting up your office email server

One problem that gets servers black listed is that small offices use the same IP address for their email server and the web traffic for the office computers. Small businesses often use a DSL service and just has one IP address and uses a small router to share that IP for several office computers.

The problem occurs when someone gets a virus that starts sending spam. The virus spam comes from the same external IP as your email server and your whole office is black listed. And it takes a lot of effort to clean yourself off everyone's black list even after you get rid of the virus. In fact - if this should happen to you it might be easier to ask your provider for a new IP rather than to try to get delisted from all the lists.

But - if you can avoid being listed in the first place that's even better.