Office 365 supports multiple domains on each subscription. This is helpful where you have subdomains, for example mycompany.com.au and staff.mycompany.com.au, or where you have mycompany.com.au and mycompany.net.au.
To add another domain, login to the Office 365 Admin Centre, as detailed here: https://neural.zendesk.com/hc/en-us/articles/217622427
Once logged in, select Domains from the navigation menu, then click 'Add a Domain'
Enter your additional domain, then click next.
Office 365 requires a domain be validated before it can be attached to a subscription. This is done by setting a DNS record on the domain, either a TXT record or an MX type record. Copy the TXT record that Office presents, similar to the below, then create a record using these details at your DNS host. If you have managed DNS, provide these details to your host.
If you're unable to create a TXT record, you can opt to use an MX record instead. To retrieve the MX record details, choose the 'Use an MX record instead' link at the bottom of the page. Office will then present you with the details necessary to create the record, as follows:
Once a record of either type has been created, choose 'Okay, I've added the record'. Office will then check that the record exists and, if successful, associates the production domain with your subscription. Click next to continue.
If the portal advises that a record can not be found, it may have been added into your DNS host incorrectly, or may still be propagating. Please double check the record, allow some time to pass, then try to verify again.
Once your additional domain has been confirmed, you can add this domain as an alias for your existing users. To do so, please follow the steps outlined here: https://neural.zendesk.com/hc/en-us/articles/217169028
If you would like to make this the default domain going forward, you can do so by returning to the domains section, selecting the new domain, then choosing Set as Default from the actions.