Unfortunately this seems to still be an issue on both macOS and iOS.
iOS Mail is unable to send using an IDN (A-label) address, macOS Mail can at least do that. S/MIME certificates with IDN (A-label) work with neither when sending.
Usage of imported certificates of other people (with IDN) also doesn't seem to work.
In general Mail seems to be very fragile when handling UTF-8 or S/MIME that it's actually dangerous (for example CVE-2023-40440 that I found).
TaaviE
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