If you are having trouble getting your SMTP server set up connected with Office365, it may be a security issue.
First check that you have your username and password correct and if you are still not able to connect, it's possible that the connection is being blocked on the exchange server's end.
Do you know if your Office365 account has a two step verification? If so, You may want to create an app password:
This app password can be pasted into the PhoneBurner SMTP Password section, instead of using your actual office365 password.
This is information from the support article:
An app password is a code that gives an app or device permission to access your Office 365 account.
If your admin has set up 2-step verification for your organization, and you're using apps that connect to your Office 365 account, you'll need to generate an app password so the app can connect to Office 365.