Bugfix invalid DKIM-signature with zero To-addresses

When using PHP's mail() function, and when there are no To-addressees (e.g. only BCC), then the DKIM-signature was not correct. There was a discrepancy between how mail() was called (empty $to) versus the DKIM-signature ('undisclosed-recipients:;').

src/PHPMailer.php

@@ -1892,6 +1892,12 @@ class PHPMailer
             $toArr[] = $this->addrFormat($toaddr);
         }
         $to = implode(', ', $toArr);
+        
+        //If there are no To-addresses (e.g. when sending only to BCC-addresses)
+        //the following should be added to get a correct DKIM-signature.
+        //Compare with $this->preSend()
+        if(trim($to) == '') { $to = 'undisclosed-recipients:;'; }
+
 
         $params = null;
         //This sets the SMTP envelope sender which gets turned into a return-path header by the receiver