TYPO3 Core

In t3lib_div::validEmail() the IDNA converter is used to prepare for international domain names; but as the conversion is applied to the whole email address, umlauts in the local part of an email address also get converted and the email address subsequently is regarded as valid though umlauts are not allowed in the local part according to RFC 5321 where only normal ASCII characters (plus some special characters) are allowed (http://tools.ietf.org/html/rfc5322#section-3.2.3).
To fix the bug, the email address has to be split and only the domain part must be converted using IDNA; the email address is then composed again and validated.