Prevent mail beeing marked as spam

I'm wondering how to prevent my开发者_开发知识库 emails from my site being marked as spam? I'm using sendmail.

I ran into this article the other day and it feels like a shame to hoard it:

How To Ensure Your E-mail Gets Delivered

It covers a good range of topics wrt. hard/soft bounces, limits, reverse-lookups, blacklists, etc. and gives recommendations for dealing with different situations.

Happy ... spamming? :)