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I understand that you would like to change the email addresses from using the domain xxx.onmicrosoft.com to customdomain.com. If there is any misunderstanding, please do not hesitate to let me know.
To change the email address, you can do the steps below:
1. Log in Office 365, click Admin.
2. Click Users on the left panel, select a user account, and click Edit.
3. Click Details, behind User name, select the custom domain, and click Save.
By doing so, the email address for this user account is changed to firstname.lastname@example.org. The mailbox data will not be lost. However, you will need to reconnect the Office 365 account to Outlook desktop client. For detailed information, you can refer to http://help.outlook.com/en-us/140/ms.exch.ecp.useoutlookanywhere.aspx.