If you suspect an issue with Office 365 messaging, it is recommended to use Microsoft Remote Connectivity Analyzer, a web application that you can access at: https://testconnectivity.microsoft.com/ , Microsoft Remote Connectivity Analyzer is a remote test of Microsoft servers on the Internet that can diagnose common connectivity issues with Office 365, On-Premise Exchange servers, Lync/OCS servers, Outlook clients, and Internet email.
Using Office 365 as an example, use the Office 365 tab of the Microsoft Remote Connectivity Analyzer to test functionality to check basic Office 365 messaging functionality, including evaluating whether the DNS configuration of the Office 365 messaging elements is properly configured as shown in the image below:
• Office 365 General Tests 1. Office
365 Exchange Domain Name Server (DNS) Connectivity Test: Checks external domain name settings, including checking for problems with mail delivery and any client connectivity issues related to DNS.
2. Office 365 Lync Domain Name Server (DNS) Connectivity Test checks the Lync-related external domain name settings for custom Office 365 domain users.
3. Office 365 Single Sign-On Test This test allows you to verify that you can sign in with on premise credentials To Office 365, this test also performs basic validation of the Active Directory Federation Services configuration.
• Microsoft exchange ActiveSync Connectivity Tests
1.Exchange ActiveSync This test checks if the mobile device can connect to Office 365 messaging resources using Exchange ActiveSync.
2.Exchange ActiveSync AutoDiscover This test checks whether the device is successfully using Exchange ActiveSync to obtain configuration settings from the AutoDiscover service hosted through Office 365.
• Microsoft Exchange web Services Connectivity Tests
1.Synchronization, Notification, Availability, and Automatic Replies check the Office 365 deployment Availability and functionality of resources in Exchange Web Services.
2.Service Account Access (Developers) checks if the service account can access Office 365 mailboxes, create and delete items in mailboxes, and access mailboxes through Exchange impersonation.
• Microsoft Office Outlook Connectivity Tests
1.Outlook Connectivity checks using RPC over http and MAPI Client connection to
Office 365. 2. Outlook Autodiscover checks the provisioning of Outlook by Office 365 settings via Autodiscover service.
• Internet Email Tests
1. Inbound smtp Email checks if inbound smtp email can be sent to Office 365 domain.
2. Outbound SMTP Email checks that the Office 365 mail domain is properly configured for reverse DNS, sender ID and RBL checking.
3.POP Email performs POP3 client email checking
on Office 365 mailboxes. 4.IMAP Email performs IMAP4 client on Office 365 mailboxes Side Email Check.
• Free/Busy Test
1. Check whether the Office 365 mailbox can access the free/busy information of the internal mailbox, and also check whether the internal mailbox can access the free/busy information of the Office 365 mailbox.
Taking the customized development for Office 365 that needs to link Office 365 Exchange Web Services as an example, it is found that there is a connection failure during the call. At this time, we can use Microsoft Remote Connectivity Analyzer->Office 365->Office 365 Exchange Web Services Connectivity Tests -> Service Account Access (Developers) to check the information, the specific operation steps are as follows:
1. Log in to the website, https://testconnectivity.microsoft.com
2. On the website interface, select Office 365-> Service Account Access (Developers), and click Next in the lower right corner, as shown in the following figure:
3. In the Service Account Access interface, enter the corresponding information, and check "Use Autodiscover to detect server settings" and the verification code, then click "Perform Test" in the lower right corner, as shown in the following figure 
4. According to the result, you can see: DNS resolution failed.
