Catch InvalidArgumentException thrown in \TYPO3\CMS\Core\Utility\GeneralUtility::idnaEncode
A string with an invalid email address `“mailto:firstname.lastname@example.org”`.
TYPO3\CMS\Core\Utility\GeneralUtility::validEmail should return false.
An exception `Conversion from UTF-8 to UCS-4 failed: malformed input at byte 8` is thrown.
The whole problem occurred in \TYPO3\CMS\Core\LinkHandling\LegacyLinkNotationConverter::resolve() during an upgrade from TYPO3 7 LTS to TYPO3 8 LTS. The link which is parsed is `<link “mailto:email@example.com”>firstname.lastname@example.org</link>`. Please don't ask how this happens, I don't know. But I wouldn't expect the validEmail() function to throw an error.