A due badge, workflow action or renewal request does not prove email delivery. Confirm that the installation has its notification sender/job configured, the intended recipient/owner is correct and the relevant job ran. Check junk/quarantine using your organisation's process. Ask the operator for delivery evidence; a prior test notification does not prove every later alert or business digest was delivered.
If HTTP 429 appears on a build with request protection, stop rapid retries and wait for the indicated retry interval. Repeated busy/timeout errors should include the time and affected action in a support request. Do not disable protection from the browser.