[Steuard]

This comment is pretty frightening to me. Email is very often used for "mission critical" purposes, not just for exchanging messages with other users (let alone just "with other users of the top 5 providers"). I know that enabling international characters is important for a whole range of reasons, and that's worth some significant costs. But email infrastructure is emphatically not something where "works most of the time" or even "works 95% of the time" is good enough. I think there really is a need for pretty substantial attention to backward compatibility in this case.