diff --git a/test/T770-emacs-subject-warnings.sh b/test/T770-emacs-subject-warnings.sh
new file mode 100755
index 00000000..f9d27140
--- /dev/null
+++ b/test/T770-emacs-subject-warnings.sh
@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+
+test_description="emacs subject warnings"
+. $(dirname "$0")/test-lib.sh || exit 1
+. $NOTMUCH_SRCDIR/test/test-lib-emacs.sh || exit 1
+
+test_require_emacs
+
+test_begin_subtest "notmuch-test-subject-warning part 1"
+test_emacs_expect_t '(notmuch-test-subject-warning-1)'
+
+test_done
diff --git a/test/emacs-subject-warnings.el b/test/emacs-subject-warnings.el
new file mode 100644
index 00000000..1e547429
--- /dev/null
+++ b/test/emacs-subject-warnings.el
@@ -0,0 +1,48 @@
+(require 'cl-lib)
+(require 'notmuch-mua)
+
+(defun subject-check-test (&optional fn)
+  "Test `notmuch-mua-subject-check'.
+Optionally, evaluate FN before doing the test.
+
+Return t if the message would be sent, and nil otherwise."
+  (notmuch-mua-mail)
+  (message-goto-subject)
+  (when fn
+    (funcall fn))
+  (prog1
+      (condition-case nil
+	  ;; Force `y-or-n-p' to always return `nil', as if the user
+	  ;; pressed "n".
+	  (cl-letf (((symbol-function 'y-or-n-p)
+		     (lambda (&rest args) nil)))
+	    (notmuch-mua-subject-check)
+	    t)
+	('error nil))
+    (set-buffer-modified-p nil)
+    (kill-buffer (current-buffer))))
+
+(defvar subject-check-tests
+  '(;; These are okay.
+    (t . (lambda () (insert "something")))
+    ;; These should not be okay.
+    (nil)
+    (nil . (lambda () (insert "         ")))
+    (nil . (lambda () (insert "		  	")))
+    (nil . (lambda () (insert " ")))	; NON-BREAKING SPACE
+    ))
+
+(defun notmuch-test-subject-warning-1 ()
+  (let (output expected)
+    (dolist (test subject-check-tests)
+      (let* ((expect (car test))
+	     (body (cdr test))
+	     (result (subject-check-test body)))
+	(push expect expected)
+	(push (if (eq result expect)
+		  result
+		;; In the case of a failure, include the test
+		;; details to make it simpler to debug.
+		(format "%S <-- %S" result body))
+	      output)))
+    (notmuch-test-expect-equal output expected)))