This is a useful little list of very sensible email testing options. I find it always best to step back and understand what are the broadest questions you need answers to. This might be as simple as who should you send your emails from and how frequently, to call to action placement or tone of voice.
It's always worth having a checklist of these test scenarios in order to make sensible decisions about developing and improving your email strategy.
Download the paper here.
This particular white paper on the subject is by Greg Grdodian of Reach Marketing