diff --git a/lisp/mail/smtpmail.el b/lisp/mail/smtpmail.el
index 43291a3028b..db0510c7e84 100644
--- a/lisp/mail/smtpmail.el
+++ b/lisp/mail/smtpmail.el
@@ -599,6 +599,8 @@ USER and PASSWORD should be non-nil."
     (when (eq (car ret) 334)
       (let* ((challenge (substring (cadr ret) 4))
 	     (decoded (base64-decode-string challenge))
+	     (password (encode-coding-string password 'utf-8))
+	     (user (encode-coding-string user 'utf-8))
 	     (hash (rfc2104-hash 'md5 64 16 password decoded))
 	     (response (concat user " " hash))
 	     ;; Osamu Yamane <yamane@green.ocn.ne.jp>:
@@ -618,8 +620,10 @@ USER and PASSWORD should be non-nil."
 (cl-defmethod smtpmail-try-auth-method
   (process (_mech (eql 'login)) user password)
   (smtpmail-command-or-throw process "AUTH LOGIN")
-  (smtpmail-command-or-throw process (base64-encode-string user t))
-  (smtpmail-command-or-throw process (base64-encode-string password t)))
+  (let ((password (encode-coding-string password 'utf-8))
+        (user (encode-coding-string user 'utf-8)))
+    (smtpmail-command-or-throw process (base64-encode-string user t))
+    (smtpmail-command-or-throw process (base64-encode-string password t))))
 
 (cl-defmethod smtpmail-try-auth-method
   (process (_mech (eql 'plain)) user password)
@@ -628,11 +632,13 @@ USER and PASSWORD should be non-nil."
   ;; violate a SHOULD in RFC 2222 paragraph 5.1.  Note that this
   ;; is not sent if the server did not advertise AUTH PLAIN in
   ;; the EHLO response.  See RFC 2554 for more info.
-  (smtpmail-command-or-throw
-   process
-   (concat "AUTH PLAIN "
-	   (base64-encode-string (concat "\0" user "\0" password) t))
-   235))
+  (let ((password (encode-coding-string password 'utf-8))
+        (user (encode-coding-string user 'utf-8)))
+    (smtpmail-command-or-throw
+     process
+     (concat "AUTH PLAIN "
+             (base64-encode-string (concat "\0" user "\0" password) t))
+     235)))
 
 (cl-defmethod smtpmail-try-auth-method
   (process (_mech (eql 'xoauth2)) user password)