diff -Naur emacs-old/lisp/mail/smtpmail.el emacs/lisp/mail/smtpmail.el
--- emacs-old/lisp/mail/smtpmail.el	2014-04-13 18:08:54.210814676 +0000
+++ emacs/lisp/mail/smtpmail.el	2014-04-13 18:06:07.502802860 +0000
@@ -580,7 +580,7 @@
       (smtpmail-command-or-throw
        process
        (concat "AUTH PLAIN "
-	       (base64-encode-string (concat "\0" user "\0" password) t))
+	       (base64-encode-string (string-as-unibyte (concat "\0" user "\0" password)) t))
        235))
      (t
       (error "Mechanism %s not implemented" mech)))))