A whole set of conventions has grown up to make email writing easier, and the resulting email easier to understand. The good thing about email is that it is quick. The bad thing is that it has to be 'basic' because not all email systems support formatted text. The writer sends an Email to one person. That question is seldom answered.
Most Email programs accept multiple addresses in a header, separated by a comma.
Subject Start with a strong subject line. The subject line is your headline. This way, the recipient will have some idea of what you are writing about before they even open the email. Message Use the inverted pyramid form of writing. Your most important statements should appear in the first paragraph.