A message, or email message, is the basic unit of electronic mail. The format of a message is defined by RFC 822. Nearly all email messages are transmitted over the internet, which means that the contents of such messages are further constrained by the SMTP protocol that is used for internet message transmission, as defined in RFC 821.
In brief, the primary constraints on an email message is that it may contain only printable US-ASCII characters, and that lines of text in the message may not exceed 1000 characters, including the carriage-return/linefeed pair at the end of each line. These constraints are fairly strict, and do not permit messages to contain text in languages other than English, or to contain non-textual data such as images. The Multipurpose Internet Mail Extensions (MIME, RFC 2045) provide a way to encode other kinds of text and data so that they can be carried in an email message. Most modern email software supports the MIME standard; one notable exception is Emacs Rmail.