So I have been using DNN locally for a while now and SMTP has been working fine. My server is hosted on site. I use an SMTP relay on my local environment that sends the email to Office 365. For Authentication I use anonymous. I am working on moving my website to a web host and so I need to change the SMTP settings. I changed it to OAuth and tried to get OAuth to work. Not having much success, but that might be another issue unless someone has good documentation. I get an error that the URL Redirect is incorrect, for some reason DNN seems to be going to Http not Https. 

Anyways the issue I encountered was after my testing I had to set my SMTP settings back to what they were before so email would work again for my modules. After changing the SMTP authentication back to Anonymous and changing the server address back I save the settings and clicked on Test SMTP settings and now this fails and doesn't send an email. Error I get now is "There is a problem with the configuration of your SMTP server. Mail was not sent. An error occurred while attempting to establish an SSL or TLS connection. The host name did not match the name given in the server's SSL certificate. The remote certificate is invalid according to the validation procedure."

I never got this error before so it appears switching my setting back may not be removing the settings I used to test. Luckily when I send a test email from one of my modules (Mandeeps LiveForms) the email goes through. But not sure if something else will be broken.