--- -	2018-02-04 01:21:02.374212479 +0000
+++ /tmp/smtpmail.el	2018-02-04 01:20:44.340497887 +0000
@@ -838,8 +838,9 @@
 		 ((and auth-mechanisms
 		       (not ask-for-password)
 		       (integerp (car result))
-		       (>= (car result) 550)
-		       (<= (car result) 554))
+		       (or (and (>= (car result) 550)
+				(<= (car result) 554))
+			   (eq (car result) 450)))
 		  ;; We got a "550 relay not permitted" (or the like),
 		  ;; and the server accepts credentials, so we try
 		  ;; again, but ask for a password first.