I wonder how necessary the content type and length changes actually were. It seems that the blacklisted IP caused most of his problems.

I'm the author and it would be fair to say that removing the IP from the Spamhaus blacklist had the biggest effect, although certain accounts (especially Yahoo) required different measures.

It's great to have a concise, consolidated guide on delivering email; it always seemed like magic to me.

I don't know why this is presented as having anything to do with a Rails App. The application doesn't factor into any of the solutions which are basic 'how to get your email delivered' tips.

I'm the author of the article. That's a fair point, although there are some application specific measures that were taken. This includes setting the mime type and headers. Basically though the advice applies for any web app.