Configuring domains in the Office 365 Hybrid Configuration Wizard
When you configure a hybrid deployment, you need to choose which of your on-premises domains you want to share between your on-premises organization and your Office 365 tenant. Sharing a domain between these two environments lets users in either one to send and receive mail sent to that domain. Also, on-premises servers and Office 365 will understand how and where to route mail sent to users in both environments.
All of the accepted domains that you've configured in your on-premises Exchange organization are listed on this page. Select the checkbox for each domain that you want to share with your Office 365 tenant. You should, at minimum, select the primary accepted domain used in your on-premises organization.
Important
The domain(s) you select need to be added to your Office 365 tenant before you continue in this wizard. If you don't add the domains first, you'll get an error when you try to complete the next step. If you want to learn more, check out Add a domain to Office 365.
If you select more than one domain, you can choose which domain is used to retrieve AutoDiscover information for your on-premises organization. In most circumstances, you won't need to do this and can leave the AutoDiscover value set to False. Only select an AutoDiscover domain if you have a specific domain that includes the AutoDiscover configuration information needed for the Hybrid Configuration Wizard to successfully configure the organization relationship between your on-premises organization and your Office 365 tenant.