Append behavior of __setitem__

Barry Warsaw writes:

 > So I'm just starting to read RFC 5322 and I'm starting by skimming  
 > over Appendix A (differences between RFC 5322 and 2822).

I know Barry's a big supporter of the Postel Principle.  As a
guideline[1], how far back should we be lenient?  RFC 822 (no leading "2"
;-)?

Footnotes: 
[1]  Presumably over time we'll accrete definitely non-conforming
practices that we need to accept and do something sane with (eg, we
can't just raise ArmageddonException because we get a header with
8-bit characters in it).  But I think we also should have a plan for
formerly acceptable syntax that has been restricted in more recent
RFCs, etc.