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.