how did the idna theory start?

[Shawn Steele]

> Why the EAI stopped using IDNA as solution of downgrading and it went to use fully UNICODE? 

The biggest challenge for IDN is that all machines have to agree on the rules for legal characters, comparison, etc.

For EAI, the local part is basically defined by the host system, so it can do the comparisons/rejections/whatever.  Other systems don't have to know which form(s) are legal or best, as the mail server will process or reject mail appropriately.

-Shawn