MUAs that goof the semantics of the "Sender:" header field are either not implementing EMail the way it has been specified by the IETF, or they goofed the second part of the above explanation for SHOULD.
I wouldn't disagree. If you can find an Internet where everyone correctly implements every RFC, I'd sure like to see it. In the meantime, we're stuck with the one we have.
-- Regards, John Levine, johnl@xxxxxxxx, http://jl.ly Please consider the environment before reading this e-mail.