Mis-spellings or typos are readily accepted and often expected. Messages often carry no salutations. It's common to ask for a message to be resent when someone has inadvertently deleted it. Using the reply form of sending a message means that you can include parts of the original message and so not need to reply in full sentences. Trying to be funny is discouraged because it is so difficult over this medium.
Junk mail is a real no no.
It is a distinct advantage to be confident in your own Email use before you start using it with students. This way you can help them when they get stuck. I set up a mailing list for each student group. There may be overlaps, but the advantage is that if you want to send a whole group message you can do this is one go rather than umpteen individual messages. The mailing list can be set up in one of two ways.