For many years IDN were correctly handled both on MacOS and iOS. This is no longer the case.
Another poster already reported on Apple discussion forum that IOS mail is no longer able to send mail from an IDN email address, always displaying an incorrect message about an "invalid address". This poster only had general incompetent advice from an Apple poster about checking his email account configuration. As this poster already reported an IDN account with worked previously no longer does. I can't date precisely which release of iOS regressed. iOS 15 for sure, 14 possibly.
Since yesterday, I spend time trying to figure out why my S/MIME certificate no longer worked on my Mac, for the same IDN email account. After many test I figured out a partial workaround :
- the S/MIME certificate is setup with the punnycode form of the IDN domain
- adding an identity preference for the certificate did no help
- but editing this identity preference by changing the punnycode form to the utf (accented, NFC form) form did help : the Mail app is now able to sign outgoing email.
- I find no way to restore the ability to encrypt email for this account neither replying to another account for which I have another certificate, nor sending encrypted mail to the IDN address from that other account.
Clearly on both iOS and MacOS there is a regression in the punnycode/utf translation/comparison. That may be due to the utf8/utf16 conundrum or normalization forms NFC/NFD.