The test will look for issues with mail delivery such as not receiving incoming email from the Internet and Outlook client connectivity issues that involve connecting to Outlook and Exchange Online. When this happens, email continues to work however, the user is not going to receive any emails to Outlook since it’s connected to Office365 they receive it to their respective source server. This test will check the external domain name settings for your verified domain in Office 365. This feature is great since it would automatically detect your Office365 account however, it can also be a huge problem when you’re in the process of migrating to Office365.