I have an IDN domain configured, let's call it "exümple.org", and a mailbox called "test@exümple.org". When I log on to SOGo (I'm using it as part of mailcow 2026-07a) and send an email from that account, the "From:" header uses the punycode representation "test@xn--exmple-4ya.org" (and not the expected "test@exümple.org" that humans commonly use and expect!).

So a recipient sees the very cryptic sender address of "test@xn--exmple-4ya.org" that will typically be unknown (and untrusted!) to them, and they will not recognize the sender as a known one.