Hello Colin,
Thank you for your question about Outlook and Gmail. If your email domain is hosted with us, you can check the mail logs to see at which point the messages may be failing. Are the messages arriving at the server? Is there a username/password, SMTP setting awry? The logs will be able to tell you. Or, if you are in a shared hosting plan you can contact our Live Support, and they can help you access those logs for your server.
Best,
Christopher M.