The only times where I ran into this problem, xn-- domains were involved, so there was a reason to do that.A separate message. My job involves finding and fixing interoperation problems.
Based on this thread, I suspect that there's a bug somewhere in the vicinity of Plesk that leads to some messages being labelled as "this message requires SMTPUTF8" even though there's no really good reason to do that. I'd like to locate the source of those message and fix that bug. If anyone sees the problem, I'd be grateful for a pointer or any relevant data.