As per HTML standards, user agents should convert punycode in the domain labels of the value to IDN in the display and vice versa. See https://html.spec.whatwg.org/multipage/input.html#email-state-(type=email)
phpBB regex does not recognize digits and dashes in punycode converted IDN email addresses, in 1st domain level in particular, f.e. admin@xn-b1agh1afp.xn-p1ai when set as "Contact email address" or "From email address" causes "The email address you entered is invalid." error in ACP.