There are 3 common choices of wording/spelling for what it is I do, and people constantly use different spellings. There's a lot of different reasoning for choosing what you use, and I've always stood behind chainmail as the proper spelling. Here's what I think about the different options.
Chainmail - To me, this is the only correct way to spell the word that means armor. It's one word, simple and straightforward, doesn't have any unnecessary fluff, and you get exactly what you think. Cool armor. ...or a RPG from Gary Gygax where people wear chainmail armor. Go with the Gygax.
Chain mail - Chain mail is what you get in your mailbox that tells you to forward a message to 45 of your closest friends or a thousand birds will fly over and poo on your car before midnight. That's no fun.
Chainmaille - This word shouldn't exist. Someone wanted to try to make chainmail fancy and put a fluffy extra 'le' on the end. The only thing this is good for is making the word more difficult to spell.
That's my two cents on it at least. What do you think? :)