I would like to confirm that for me the problem I am facing is linked with the x-mailer.
I downloaded version 7.0.1 and commented one line in the file mailer.php in the includes folder.
Original line
$this->mailer->XMailer = ‘ ‘; // A space!
Modified line
/* $this->mailer->XMailer = ‘ ‘; // A space!
*/
And with this change my mails are not rejected anymore
Best regards