+test_begin_subtest "Support single-message mbox (deprecated)"
+cat > "${MAIL_DIR}"/mbox_file1 <<EOF
+From test_suite@notmuchmail.org Fri Jan 5 15:43:57 2001
+From: Notmuch Test Suite <test_suite@notmuchmail.org>
+To: Notmuch Test Suite <test_suite@notmuchmail.org>
+Subject: Test mbox message 1
+
+Body.
+EOF
+output=$(NOTMUCH_NEW 2>&1)
+test_expect_equal "$output" \
+"Warning: ${MAIL_DIR}/mbox_file1 is an mbox containing a single message,
+likely caused by misconfigured mail delivery. Support for single-message
+mboxes is deprecated and may be removed in the future.
+Added 1 new message to the database."
+