In a hybrid environment, some of your organization's mailboxes are on an on-premises Microsoft Exchange server. Other mailboxes are in Microsoft Exchange Online. This is always the case if you migrate all your organization's mailboxes to Microsoft Exchange Online (Office 365).
You can find more information about hybrid operation in the Exchange documentation for hybrid deployments on the Microsoft Docs website.
You must perform the following preparatory tasks to ensure that access to the mailboxes works properly:
Set up the autodiscover function in Microsoft Exchange.
Make sure that the on-premise Active Directory is synchronized with the Azure Active Directory or with Azure AD Connect. You can find more information about Azure AD Connect on the Microsoft Docs website by searching for "What is hybrid identity with Azure Active Directory?".
Make sure that EWS impersonation users with the same password can log in on the on-premises Microsoft Exchange server and Microsoft Exchange Online (password synchronization).
If you have configured the Groupware app for the hybrid environment, the appropriate Microsoft Exchange server (on-premises or Microsoft Exchange Online) is dynamically determined for each mailbox. Since the determination can take several seconds, the assignment of the mailbox to Microsoft Exchange Server is stored for twelve hours and then redetermined. If you restart the Groupware app, all the stored mappings are deleted immediately.
You can also simply configure multiple Microsoft Exchange connections for the same domain. The connection for a mailbox is then automatically determined from the existing mailboxes. This also gives you the advantage of being able to mix the EWS API and Microsoft Graph API. For instance, you can connect the Microsoft Exchange server (on-premises) using the EWS API and basic authentication. On the other hand, you can connect Microsoft Exchange Online using the Microsoft Graph API and OAuth 2.0.