use a string that will never match.

diff --git a/lisp/mail/sendmail.el b/lisp/mail/sendmail.el
index 5a01eae58d5563f961697ec1df3f15f09d8a69dc..85d5690dbe40faa66db639460d9dfeb21bfcd013 100644 (file)
--- a/lisp/mail/sendmail.el
+++ b/lisp/mail/sendmail.el
@@ -272,7 +272,7 @@ actually occur.")
            ;; Use EVAL to delay in case `mail-header-separator' gets changed.
            '(eval .
              (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.