Suppose you receive mail from my Unicode address. When you compose your
reply, you do not know whether my SMTP server accepts an a-label form
of any domains in your reply, so when you want to formulate a Unicode
domain in your reply (header or SMTP), you cannot write if/then logic
to pick the best for each recipient, you have to choose.
653x is
compatible with 589x in the sense that both can be used. But
incompatible, since they are different solutions to the same problem,
and you have to use exactly one for each instance of the problem.
You
can use both if you e.g. use u-labels for RCPT TO and a-labels for the
MAIL FROM (unless you need Unicode for the localpart), but that is IMO
silly. In reality you have to choose, and that makes them
incompatible.
You could leave the IDNA code in and let people chose at
compile time, of course.
Arnt
