It appears that Nicolas Fiumarelli via UA-EAI <nicocamarao@gmail.com> said:
I have a doubt regarding the handling of EAI (Email Address Internationalization) in intermediary email servers that do not fully support EAI, particularly those that do not advertise SMTPUTF8.
If an email is sent to an EAI-enabled recipient but passes through a forwarding server that does not support SMTPUTF8, how should this be handled?
There is no such thing. If the intermediate server does not support SMTPUTF8, the attempt to deliver to that server fails and the message bounces.
Is it enough for the forwarding server to simply relay the message while keeping SMTPUTF8 intact, or would it cause delivery failures?
Failure.
Are there best practices or alternative mechanisms for ensuring proper delivery when intermediary servers do not comply with SMTPUTF8?
The experimental version of EAI mail tried to provide for fallback to ASCII addresses. That did not work and we took it out of the final version of EAI. If all the servers in the path handle SMTPUTF8, the mail works. Otherwise it doesn't. R's, John