I found and issue that causes Apple Mail in Sierra to hang when there are typos. Can someone else confirm this as being a Sierra bug? I have experience on two Macs, one being a clean install with nothing on it. Paste the follow gibberish text below in a new email and then continue typing gibberish and see if after adding 2-3 more sentences it starts to stutter and freeze. To solve it for now, you need to go into Preferences>Keyboard>Text and for spelling change it from Automatic to U.S. English. I guess when its set to automatic the system is having trouble decerning what language it is when there are typos. I was actually able to replicate the issue after adding 2 typos in a email. If there are no typos, its fine.