I just got a really good education on how to make sure your (legit) email will navigate common spam blockers and be delivered successfully, thanks to Jeff Atwood.

Summary

1. Make sure the computer sending the email has a Reverse PTR record. Your ISP has to do it, not your DNS provider or web host.
2. Sign your messages using DomainKeys Identified Mail. Requires DNS and code changes.
3. Set up a SenderID DNS record. Far less critical than the first two, but still nice to have.

Did it work?

1. Send a message to a GMail account–they provide excellent diagnostic headers. Look for Received-SPF and Authentication-Results.
2. Use the Port25 diagnostic service (check-auth@verifier.port25.com). You can ignore a DomainKeys check fail if the DKIM check passes.