It’s a good idea to add users to your Office 365 subscription while you’re setting up your custom domain with your own email, so they’ll keep getting email sent to email addresses that use your domain name.
The short story:
Why add or update users while you're setting up your domain? When you change your email to Office 365, by updating your domain's MX record in the next step, ALL email sent to that domain will start coming to Office 365. If other people use your domain for email, you must set up Office 365 mailboxes for each of those people. Otherwise they'll lose any email sent to them at your domain when you switch email for the domain over to Office 365. (Confusing? Learn more about how domains work with email.)
Tip: Do you just want to switch a few email addresses to Office 365? You can take steps to pilot Office 365 with just a few email addresses instead.
To start using your domain with Office 365
Sign in to Office 365 with your work or school account.
On the Manage domains page, choose Add domain.
This starts the wizard, which walks you through adding your domain to your Office 365 subscription, switching your user ID (which is also your email address) to use your domain name, and setting up your users’ email addresses with your domain.
Watch a short video that shows the steps in the wizard: Video: Set up your domain in Office 365.
Why add users now?
You can add users to Office 365 at any time. Why do it now? If you already have users getting email at your custom domain name, if you add the users and set up their addresses to use your custom domain before you finish setup, there will be no disruption in mail flow. This provides continuity for the people in your organization, for your customers, and others.
Tip: If you created users in Office 365 before you added your domain, we’ll help you switch those users to email addresses that use your domain name. You don’t have to add them again.
What if I don’t add users who have email addresses that use my domain?
If you haven’t added an email address that uses your domain name before you set up DNS records for your domain in Office 365, you’ll disrupt that user’s email. To set up your domain with Office 365, you either change your nameserver records so that Office 365 manages your DNS records or you set up an MX record at your DNS host to point to Office 365. After you change the records, any messages sent to email addresses on your domain must have a mailbox in Office 365. If you haven't added every email address on your domain to Office 365, email won’t arrive—not in the person’s current email system and not in Office 365.
So be sure to add all email addresses that use your domain name to Office 365 before you change the records!
In sum: until you switch over email by either changing the nameserver records or the domain's MX record to Office 365, people will continue to receive email sent to the email addresses wherever they currently get email. Once you switch the records, all email sent to your domain will begin coming to Office 365.
Learn more about how email works
How does the Internet figure out where to deliver email messages? Watch this short video to learn more!