diff --git a/notmuch-show.c b/notmuch-show.c
index 7fb40ce9..8c23f821 100644
--- a/notmuch-show.c
+++ b/notmuch-show.c
@@ -1399,6 +1399,14 @@ notmuch_show_command (notmuch_database_t *notmuch, int argc, char *argv[])
 	    fprintf (stderr, "Error: specifying parts is incompatible with mbox output format.\n");
 	    return EXIT_FAILURE;
 	}
+	if (params.crypto.decrypt != NOTMUCH_DECRYPT_FALSE
+	    && params.crypto.decrypt != NOTMUCH_DECRYPT_AUTO) {
+	    fprintf (stderr, "Warning: mbox format does not support decryption (ignored)\n");
+	}
+	if (params.crypto.verify) {
+	    fprintf (stderr,
+		     "Warning: mbox format does not support signature verification (ignored)\n");
+	}
     } else if (format == NOTMUCH_FORMAT_RAW) {
 	/* raw format only supports single message display */
 	single_message = true;
diff --git a/test/T520-show.sh b/test/T520-show.sh
index 8121c3db..1f7f6aa8 100755
--- a/test/T520-show.sh
+++ b/test/T520-show.sh
@@ -17,6 +17,25 @@ notmuch show foo..
 exit_code=$?
 test_expect_equal 1 $exit_code
 
+test_begin_subtest "warning for --mbox --decrypt"
+notmuch show --format=mbox --decrypt=true '*' 1>/dev/null 2>OUTPUT
+echo $? >> OUTPUT
+cat <<EOF > EXPECTED
+Warning: mbox format does not support decryption (ignored)
+Warning: mbox format does not support signature verification (ignored)
+0
+EOF
+test_expect_equal_file EXPECTED OUTPUT
+
+test_begin_subtest "warning for --mbox --verify"
+notmuch show --format=mbox --verify '*' 1>/dev/null 2>OUTPUT
+echo $? >> OUTPUT
+cat <<EOF > EXPECTED
+Warning: mbox format does not support signature verification (ignored)
+0
+EOF
+test_expect_equal_file EXPECTED OUTPUT
+
 test_begin_subtest "notmuch show --sort=newest-first"
 notmuch show --entire-thread=true '*' > EXPECTED
 notmuch show --entire-thread=true --sort=newest-first '*' > OUTPUT