On Wed, Sep 14, 2016 at 5:52 PM, Lyndon Nerenberg <lyn...@orthanc.ca> wrote: > Not the standard, but a convention. > > The standard is documented in RFC 5322 section 3.6.4 (and dates back to > RFC822).
I think you may mean RFC 5322 section 3.6.5, which does give a "MAY" suggestion for the use of "Re: " at the start of the contents of the subject field, but I'm unable to find any mention of the square bracket or "was" convention for subject line changes within a thread anywhere in RFC 5322, RFC 822, or even RFC 733. Aside from the "Re: " convention, all three RFCs describe the Subject field as unstructured text, subject to the line folding rules (defined in RFC 5322 section 2.2.3 "Long Header Fields").