I've never seen it spelled out, so I had to look it up, but Mrs. is Missus or Missis, according to Wikipedia. But to repeat, I have never seen it written out, so I don't recommend it (at least if you are writing to someone in the U.S. I don't know about the U.K.).
Misses sounds like two unmarried women: Misses Jane and Elizabeth Bennett.
Go with Ms or Ms. in any business situation.
(Also, in the U.S. South, Miss Ann or Miss Mary is a way to refer to an adult woman by a child (or by anyone if she is elderly but a family friend), regardles of her married state, but it sounds more like "Miz." My grandmother was "Miz Pearl" to many people around town.)