diff --git a/lisp/mail/sendmail.el b/lisp/mail/sendmail.el
index 76c3baf4727..0d3eeecd8c8 100644
--- a/lisp/mail/sendmail.el
+++ b/lisp/mail/sendmail.el
@@ -955,7 +955,10 @@ the user from the mailer."
 	      (error "Invalid header line (maybe a continuation line lacks initial whitespace)"))
 	    (forward-line 1)))
 	(goto-char opoint)
-	(when mail-encode-mml
+	(when (or mail-encode-mml
+                  ;; When we have long lines, we have to MIME encode
+                  ;; to get line folding.
+                  (mm-long-lines-p 1000))
 	  (mml-to-mime)
 	  (setq mail-encode-mml nil))
 	(run-hooks 'mail-send-hook)