Variables suggest ways in which this message may differ predictably from other samples of CMC, e.g.:
a synchronous chat message from N to S (synchronicity)
a public message posted by N to a listserv to which S is a subscriber (public vs. private)
private e-mail between teenage female acquaintances, or between academic male acquaintances (age, gender)
an e-mail with a different purpose, e.g. N rejecting a paper written by S for publication (purpose)
an e-mail about the conference from N to S in which personal topics are discussed (topic)