Allow umlauts in email address
TYPO3 LTS v8 and v9 require Swiftmailer with version dependency ~5.4.5. It's not possible to send emails with umlauts in local or domain part. Swiftmailer does support umlauts (both local-part and domain) from version 6:
RFC is still an proposed standard from 2012, but the use of domains (and local email part) with umlauts is steadily growing.
Updated by Christian Eßl about 2 years ago
Still an issue in TYPO3 10, although a switch to Symfony Mailer / Mime was made.
Both components look like they are still in experimental state. - For now, the Mime Component only implements a IdnAddressEncoder that does not support non-ASCII characters in the local-part. Although the class mentions a Utf8AddressEncoder, it doesn't exist in the package and the Symfony\Component\Mime\Address class has IdnAddressEncoder as a hard depedency. (You can't simply inject your own AddressEncoder without having to extend the whole class...)