The client has two domain names, username@domainA.com and username@domainB.com. Currently, the default login name is username@domainA.com. They would like to change the default login to username@domainB.com. This video shows how to change the Office 365 default login domain name.
1.Change the default email address
•Login Office 365 admin account.
•Go to Admin>Exchange admin center.
* Click on mailboxes under recipients
* Double-click on the user you want to change the default email address.
* Click on email address
* Setup default email address using SMTP type.
Note: If you have on-primer Active Directory and can’t make the change on Office 365, login your local Domain Controller to setup the default email address.
* You do that by going to User’s Properties>Attribute Editor>ProxyAddresses
* Search the user you want to make the change.
* Click on the user.
* Click on Manage username
* Change the user’s domain and click on Save the change.
* You should receive Username updated popup which confirms the username has been updated from user@DomainA.com to user@DomainB.com
Also please refer to this post: How to change AD default login email address
Please view this step by step video:
