Index: wp-includes/class-smtp.php
===================================================================
--- wp-includes/class-smtp.php	(revision 12813)
+++ wp-includes/class-smtp.php	(working copy)
@@ -604,7 +604,7 @@
       return false;
     }
 
-    $useVerp = ($this->do_verp ? "XVERP" : "");
+    $useVerp = ($this->do_verp ? " XVERP" : "");
     fputs($this->smtp_conn,"MAIL FROM:<" . $from . ">" . $useVerp . $this->CRLF);
 
     $rply = $this->get_lines();