Ok, found what could be causing that. Try changing file 'library/swift/lib/Swift.php' line 1624 to:
public function buildMail($to, $from, $subject, $body, $type='text/plain', $encoding='8bit', $return_data_part=true, &$complete_mail)
{
if ($this->message_id) {
$msg_id = $this->message_id;
} else {
$id_right = substr($from, strpos($from, '@'));
if (strpos($id_right, ">") !== false) {
$id_right = substr($id_right, 0, strpos($id_right, ">"));
}
$id_right = preg_replace('/[^a-zA-Z0-9\.\!\#\/\$\%\&\'\*\+\-\=\?\^\_\`\{\|\}\~]/', '', $id_right);
$msg_id = "<" . gen_id() . "@" . $id_right . ">";
}
$date = date('r'); //RFC 2822 date