I therefore wonder if this isn't something we ought to consider
"relaxing" in 2821bis.
If BATV really works everywhere let's do this in 2821ter, please.
I consider 2821bis as "frozen" modulo some DISCUSSes. A general
local part tag syntax with a tag registry should not be limited
to the BATV purposes, it should be open for other applications,
EAI, SRS, SES, whatever. The SASL RFCs also offer one or two
very simple mechanisms, and some rules to register more complex
stuff in separate RFCs. Obviously following the MIME model...