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.
Writing business letter can be very challenging. It is, sometimes, hard to predict how your letter is being perceived. It is better to be more formal to avoid offending the recipients. Often, people write in informal, conversational tones in business letter. This can be seen as inappropriate and can easily be misunderstood. The recipients may view it as not professional. This also includes the use of emoticons. Other ways that may trigger misunderstanding are all caps. Some may think that you are shouting at them. To avoid miscommunication, you should always spell check and proof-read your letter. Try to use formal language and address the recipients as Mr. or Ms. Do not use all caps or other inappropriate fonts, color, or punctuations.