1 files changed, 6 insertions, 1 deletions

diff --git a/lisp/mail/sendmail.el b/lisp/mail/sendmail.el
index 61c0de48613..52a9abefe65 100644
--- a/lisp/mail/sendmail.el
+++ b/lisp/mail/sendmail.el
@@ -213,7 +213,12 @@ actually occur.")
           (insert cc "\n")
           (fill-region-as-paragraph address-start (point-max))))
     (if in-reply-to
-        (insert "In-reply-to: " in-reply-to "\n"))
+        (let ((fill-prefix "\t")
+              address-start)
+          (insert "In-reply-to: ")
+          (setq address-start (point))
+          (insert in-reply-to "\n")
+          (fill-region-as-paragraph address-start (point-max))))
     (insert "Subject: " (or subject "") "\n")
     (if mail-default-headers
         (insert mail-default-headers))