Instead of just looking for a few spammy words in your message, we actually send your email to real servers that run the most common spam filters. We assess whether or not these spam filters will block your email, and why. Each spam filter is set to different levels, like Low, Medium, and High.
If your email gets blocked, you can click the "reasons" link next to each spam filter to find out why. You'll see hints about what the spam filter detected, and how much that criteria contributed to your overall "spam score." Usually, the higher your score, the more spammy things they found, and the more likely your email will get blocked. Sometimes the spam filters are pretty descriptive about what you should change. Sometimes they're a little cryptic.
Overall, there's no need to spend a lot of time getting every little item on their list fixed. As long as you pass, you're okay. But if you see something that's got a very high score (over 5 full points is really bad, anything under 3 is a good score) you might want to address that issue before sending.
Desktop Filters are used primarily by people checking email at home.
Server Filters are used by companies, so if your recipients check email at work, you'll need to get through server filters.
Gateway Filters are used by ISPs and Large corporations.