1 files changed, 4 insertions, 2 deletions

diff --git a/lisp/mail/sendmail.el b/lisp/mail/sendmail.el
index b8e36ee1517..5a01eae58d5 100644
--- a/lisp/mail/sendmail.el
+++ b/lisp/mail/sendmail.el
@@ -271,8 +271,10 @@ actually occur.")
               (1 font-lock-comment-face) (2 font-lock-type-face nil t))
             ;; Use EVAL to delay in case `mail-header-separator' gets changed.
             '(eval .
-              (cons (concat "^" (regexp-quote mail-header-separator) "$")
-                    'font-lock-warning-face))
+              (let ((separator (if (zerop (length mail-header-separator))
+                                   " "
+                                 (regexp-quote mail-header-separator))))
+                (cons (concat "^" separator "$") 'font-lock-warning-face)))
             ;; Use MATCH-ANCHORED to effectively anchor the regexp left side.
             `(,cite-chars
               (,(concat "\\=[ \t]*"