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Can’t connect? Always check the Service Health Dashboard first – if there is a service issue in progress, the solution may be out of your hands. If there isn’t a service issue, you can use these troubleshooting tips to help find the source of the problem.
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.
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 tool"). 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.
Solutions for the most common issues with Outlook are listed below. You can also use the Outlook connectivity troubleshooter to find a solution, including some that are not shown here.
Also see Fix outlook connection problems after Office 365 upgrade
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.
There is more than one possible cause to this symptom. The most common solutions are listed here.