TYPO3.SwiftMailer

Simple to reproduce:

$mail = t3lib_div::makeInstance('t3lib_mail_Message');
/* @var $mail t3lib_mail_Message */

$mail->setFrom(array($fromEmail => 'Cyrillic name like Петро - not OK'));
$mail->setTo(array($mail => 'Cyrillic name like Іван - not OK'));
$mail->setBcc(array('support@ruhomi.com', 'booking@hotelsbooking.com.ua'));
$mail->setSubject('Cyrillic subject - OK');
$mail->setBody('Plain cyrillic text - OK', 'text/plain');
$mail->send();

It nicely "delivers" into local file, but silently fails delivering via mail(). Not sure, but can be buggy for whole Unicode charset not only cyrillic area.