User account creation/edit page doesn't allow email addresses with international domain names with advanced characters like umlauts ä,ü,ä or ß. (Version in Screenshot is 8.02)
According to https://en.wikipedia.org/wiki/Internationalized_domain_name and https://en.wikipedia.org/wiki/International_email there are valid email addresses like 'max@maßanfertigung.de' or 'max@müller.de'.
Current form validation RegEx doesn't allow these email addresses.
Creating a User via the REST API doesn't have a check - User is created with an email containing extended UTF characters.