> C2) As per 5), we cannot drop "Precedence:" quite yet (unless someone
> digs up evidence that the bulk of Exchange servers on duty - used by
> tech people, at least - now support the new header)

Exchange 2010 is claimed to support RFC 3834 (i.e. likely honors the new
header):
https://technet.microsoft.com/en-US/library/ff848256(v=exchg.141).aspx .

According to
http://www.securityspace.com/s_survey/data/man.201603/mxsurvey.html ,
however, the vast majority of installations still use Exchange 2003 (see
http://capitalhead.com/articles/microsoft-exchange-server-build-numbers-and-release-dates.aspx
for build number reference. Note that the builds in the analysis are
smtpsvc.dll builds which still roughly correspond to those of Exchange).