Thanks! This is a great walk-through for those using a solution with only 1 or a few IP addresses associated with little chance of that IP changing. Do you have any experience for those using services such as Amazon SES or Google Apps?

Setting up the PTR records for reverse DNS lookups are very tricky since Amazon and Google are always rotating the IP addresses for their mail servers. We've been fighting with this in our application for a little while now (using Python, not Ruby).

We're getting about 90% of our emails through, but some are getting denied for no reason whatsoever. The worst part is, they are server-side denials, so the emails aren't even going into the spam folders of the users.

Also, I second adding the DMARC section. DMARC is extremely simple, and it's very easy to implement gradually.