TYPO3 Core

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...)