Readdle's Spark for iPhone is an awesome email client and I can't wait to use it on my iPad (coming soon) and the Mac (it's in the works). Today's update makes it even better.
In Spark 1.5 for iPhone and iPod touch, you can now customize which swipes are available in notification actions for new messages. You can also designate an email signature to be your default on a per-account basis, zoom in on HTML emails and more.