The Email validator allows umlauts within the local part. As of RFC-2822 the local part consists of dot-atom which starts with atext. In atext there are no umlauts allowed, only ALPHA, DIGIT and some special characters.
ALPHA is defined in RFC-2230 as

ALPHA          =  %x41-5A / %x61-7A

so an email address ürsula@test.xy will be validated as valid but it shouldn’t