It has been my observation that formality is used to indicate the inability of a correspondent to make a reply. Take three situations where someone is not free to respond:
If you and the Queen of England have tea, one of you might ask about the other's health, but both of you are socially constrained from actually discussing recent surgeries.
Thomas Jefferson is dead. It is not possible to ask him what the proper interpretation of the phrase "high crimes and misdemeanors" is.
If every member of a large audience tried to comment on a speech, there would be bedlam.
Conversations involving people with exaggerated status differences and those to audiences that are unborn, dead, and/or large tend to use very formal language.
Conversely, intimate discussions use very informal language. If you used the same language with your spouse that you used with the Queen, your spouse would probably wonder what he or she did to make you angry!
Thus you can control to some extent how many responses you get to your email messages by how formal your language is. Because email is so easy to respond to, people naturally tend to use very informal prose.
The informal tone encourages your correspondents to respond. This can be a very good thing if you want feedback. However, if your email address is in a very public place, you may well find yourself getting far more email than you are interested in.
So be cautious about the tone of your messages. If you want people to respond, be chatty and informal. But if you want to discourage people from sending you email, you should write much more formally.
