In an article called "Boosting Email Deliverability" Wendy Roth form Lyris Technologies explains how to ensure your email isn't mistaken for spam. ISPs don't only block emails based on the content in your subject line and body, but they may be blocking your mail before they even see the subject line. Why? Because they can't be sure that you are who you say you are.
Helping ISPs verify your identity, therefore, is a crucial part of ensuring that your email messages make it to the recipient's inbox. By using a static IP address, having both forward and reverse DNS set up for your domain name, and adding SPF information to your record, you're letting the ISPs know that you're one of the "good guys," which makes it far less likely that your email will be mistaken for spam.