When sending messages both internally or externally, we should always proof-read and spell check. In my opinion, it's always safer to use formal language, unless you know your colleagues very very well. When sending formal offer, notice, and other important documents, a formal letter should be used and it should address to the right person. Email is widely used and it is being considered as business correspondence in my workplace. So, formal language should also be used (though the language could be less formal depending on who is reading them). Memo should only be sent internally for small matters.