Different standards used in GeneralUtility::validEmail (RFC 3696) and swiftmailers Swift_Mime_Grammar-class (RFC 2822)
Tested in 6.2.18, but may also occur in current versions.
At the moment GeneralUtility::validEmail() is following RFC3696, where as the internally used swiftmailer is follwing RFC 2822.
We had a case where an email with the format "user@¹gmx.at" was checked as valid by TYPO3s internal function but lead to an Swift_RfcComplianceException when sending an email.
The problem may or may not occur because of the different standards that were used. Currently this would mean, that as a TYPO3 developer you would always have to check against the \Swift_Mime_Grammar instead of GeneralUtility::validEmail() to be on the safe side, if the email at some time should be sent by TYPO3s internal methods.