There used to be a bug in Outlook and Outlook Express's handling of uuencode (remember that?) that meant if you began a message with "begin" and two spaces, it would render the rest of the message unreadable. A few people on the WLUG mailing list tried using "begin quoted message from X", until we politely pointed out to them that some people are forced to, or choose to, use a Windows mail client, and it's bad advocacy to stop them from being able to read your messages.
Other variations on the theme pop up every now and then; here's one on a message from jdub this morning, with a little less angry and a little more style.
If you're interested, it is put there by X-Message-Flag. And it's easy to do.