a. encode domain names in IDNA
b. encode local-parts in an encoding that is to be defined, but which
hopefully is very similar, perhaps identical, to IDNA
The problem here is that nameprep is stricter than mailbox naming has
historically allowed for. Specifically, the use of mandatory lowercase
characters in nameprep will prohibit mixed-case local-parts.
Good point. OTOH, if we defined a special version of nameprep for
local-parts to allow mixed-case, then local-parts of internationalized
email addresses would be subject to case-sensitive comparison.
Of course, strictly speaking, interpretation of the local-part is up to
the domain, but in practice, case-insensitivity is often taken for
granted by both users and software.
Keith