After you add your custom domain, get Office 365 email in Outlook
After you add a custom domain for your organization (like fourthcoffee.com), you can send and receive email with the domain (like firstname.lastname@example.org) right in the Office 365 portal. Sign in to the portal, and click Outlook.
But if you also want to use Office 365 email with your custom domain in your Outlook program on your desktop, you and others in your organization can do that too. Follow the steps to set up Office 365 email with your custom domain in Outlook 2010 or Outlook 2013.
Before you set it up, make sure your Office 365 email is working with your domain
For Office 365 email on your custom domain to work with Outlook on your desktop, email first has to work correctly in the service. If you can’t send and receive email in the Office 365 portal, check for these possible issues. After email works in the portal, try again to set up Outlook with your custom email address.
If you chose (in the domains setup wizard) to have Office 365 manage your DNS records: When you added your custom domain, did you finish all the steps in the wizard, including completing the process to change your name server (NS) records to Office 365? That final step allows Office 365 to set up email with your domain for you.
If you’re managing your domain's DNS records yourself: Did you set up the MX and CNAME records for Exchange Online at your DNS host (like GoDaddy)? They’re required for email to work. (Learn how to set up the email records at many popular domain registrars.)
If you’re having trouble getting email to work in the Office 365 portal: If Office 365 email doesn’t work in Office 365, it probably won’t work in Outlook on your desktop either. Track down the Office 365 issues first (see the steps in the Track down problems section below), and then set up Outlook with your Office 365 email address.