Electronic mail is known for its informality. This is partly because of
the hacker culture whence email springs and partly because email is so
easy to send. In some ways email has more in common with phone discussion than with paper memos, which are traditionally more formal in
tone.
Because email communications are written, they can be much more
detailed than a phone conversation. And they're delivered almost
instantaneously, rather than overnight. So colleagues or friends can
have a long "conversation" -- with a written record of what they said --
over the course of a day.