In the previous installments (see: part 1 & part 2) we looked at software changes in mail servers, and in the software that lets user mail programs pick up mail. What has to change in the user mail programs?
Keeping in mind that I am far from a usability expert (my ideal interface is a model 33 Teletype), there are a few things that I can describe without going into the details of exactly how they would look.
The first and most obvious is that users have to be able to enter the addresses. This is not as obvious as it might immediately appear. While I know how to configure my keyboard to type accented letters like the ones in josémartí@gob.cu, I haven't a clue how to type 毛泽东@政府.cn, if I wanted to send mail to someone who'd given me a business card with that address. Perhaps cards will add QR codes. Or perhaps internationalized addresses will mostly be used in communities that are sufficiently homogeneous that they'll all know each other's languages and it won't be enough of an issue to need a solution.
Typing the message shouldn't be a problem, since people will presumably write messages in languages they understand. The next issue is who gets what format of mail. With EAI mail, either a recipient can totally handle it, or not at all, with nothing in between. So if you send an EAI message to a non-EAI address, it won't be delivered. If someone's address includes non-ASCII characters, that means it has to be on an EAI mail system, but for ASCII addresses, you can't tell, unless you have a hint, such as an EAI message that they sent to you. (At some point, I'll upgrade my mail to handle EAI, but my address, which I've had since 1993, won't change.)
This suggests that the address books in mail programs will try and remember which addresses can accept EAI mail, much as a decade ago they remembered who accepted HTML mail. In principle this is a huge ugly hack, in practice I expect it'll work pretty well, send ASCII mail to ASCII addresses and EAI mail to EAI addresses, updating the address book whenever you get an EAI message from an ASCII address.
It's also likely that people with EAI addresses will continue to have ASCII addresses for a long time, so non-EAI mail users can continue to correspond with them. Hence address books are also likely to remember that firstname.lastname@example.org is the same person as 毛泽东@政府.cn, so it should use the EAI address in EAI mail, but the ASCII address in messages also sent to non-EAI recipients. Again, a hack in principle, probably workable in practice.
I gather that some large mail systems in China are implementing EAI mail now, so we'll probably know soon how whether mail systems do all this. as opposed to just saying if you want to send me mail, find an EAI mail system.