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The Domain Name System (DNS) is the directory for the Internet. You have to create records in DNS so that other computers on the Internet can find services associated with your domain. Different services are represented by different types of records. Those services include email, instant messaging, and websites. Even the most experienced admins can struggle with DNS and missing or incorrect DNS records are a common cause of all kinds of problems.
The topic External DNS records describes all the different types of DNS records used by Office 365. Before you start troubleshooting, you should know where your DNS records are hosted and how to make or request changes. Learn how to track down this information.
See Help me add and verify my domain. You have to create a special record at your DNS hosting provider to prove that you own a custom domain before it can be used with Office 365. Your DNS hosting provider is typically the same as your domain registrar.
Video: Confirm ownership of your custom domain name
Video: Change DNS settings at your domain registrar
There may be a problem with your MX record. You can use an online tool to look up all the MX records associated with your domain (search for DNS lookup tools). You should see an MX record for each domain you use to receive email, and it should match the MX record that is displayed in the Office 365 admin center exactly. There should be no “extra” MX records, which could cause email to be delivered to the wrong place.
Go here to view the MX records for your organization. Select the custom domain, then click View DNS settings > View DNS records.
Note: if you're having trouble receiving mail from only one domain, it’s possible that the other organization has set up a connector specifically for sending email to you, and that they’ve set that connector to go to a specific IP address. They should update the connector to use DNS resolution instead.
Check for the existence of the SPF record. The SPF record is used to prevent phishing and spoofing. Organizations that receive email from you can check for this record to make sure that email was sent from a trusted source.
Make sure that you have associated your domain name with your public website and you have set up a DNS record so that Internet users can find you. If you’re using Office 365 Midsize or Enterprise, go here for help.
Office 365 Small Business customers should start at Set up and manage domains.
Video: Set up Office 365 to work with your website
Outlook connectivity issues can be frustrating, especially when you have a bunch of upset users who expect you to get it fixed now. the good news is that while you're working on the problem, users can continue to connect to Outlook Web App to send and receive email, schedule meetings, and lookup contacts.
The most common causes of this problem are a missing or misconfigured Autodiscover DNS record or an Outlook profile that is configured manually. Exchange Online supports Outlook connectivity only through Autodiscover. Just like it sounds, this DNS record lets Outlook automatically discover the location of your mailbox. If your Outlook profile is set up to use a specific server name, your connection can break if the mailbox is moved, such as during an upgrade. To see if this is your problem and get it fixed, follow these steps:
Need more detailed steps? See Correcting Autodiscover and DNS settings.
Outlook uses a process called Autodiscover to automatically find your settings and set up an Exchange connection to your account. Make sure that you’ve set up the CNAME record for Autodiscover. It’s shown in the DNS settings for your domain. Then follow the steps to Set up email in Outlook 2010 or Outlook 2013.
View the DNS records that are associated with Lync Online and their purpose. Are they all set up? Also take a look at this wiki article for deeper network troubleshooting.