From their website:
DNSWL.org provides a Whitelist of known legitimate email servers to reduce the chances of false positives while spam filtering. We effectively divide the entire Internet up (by IP) into five categories, and provide you with efficient access methods to this data for free.
| Trust Level | Description |
|---|---|
| High | Never sends spam. |
| Medium | Extremely rare spam occurrences, corrected promptly. |
| Low | Occasional spam occurrences, actively corrected but less promptly. This is the default for most categories. |
| None | Legitimate mail server, may also send spam. This is the default for some categories (eg Email Marketing Provider). |
 I added two of my IPs to the DNSwl list and I have had less complaints about undelivered email. Coincidence? Who knows, but it can’t hurt to add your IP to their list. Goto http://www.dnswl.org/ for more information.
Note: You’ll need to have a foreward entry setup in order to register the IP at dnswl.
