openssl genrsa -out example.com-private.pem 1024 -outform PEM
openssl rsa -in example.com-private.pem -out example.com.pem -pubout -outform PEM
$mail->DKIM_domain = 'example.com';
$mail->DKIM_private = '/path/to/my/private.key';
$mail->DKIM_selector = 'phpmailer';
$mail->DKIM_passphrase = '';
$mail->DKIM_identity = $mail->From;