Looking back at Linus' original email, I find it interesting that there's a "Summary" header in addition to "Subject:"

  > Subject: What would you like to see most in minix?
  > Summary: small poll for my new operating system

RFC 5322 [1] doesn't have anything to say about a summary header, so maybe it was an invention of Linus'.

[1] http://tools.ietf.org/html/rfc5322#section-3.6