OutlookWebAppredirection allows for redirection from the onpremisesOutlookWebAppenvironment to theOffice365OutlookWebAppenvironment. You specify a target URL for your organization (for example,www.outlook.com/contoso.com).
MailTips, out-of-office messages, and similar features understand that Office365 and on-premises users are part of the same organization.
Delivery reports and multi-mailbox search work with users who are on-premises and those working in ExchangeOnline.
Authentication headers are preserved during cross-premises mail flow, so all mail looks and feels like it is internal to the company (for example, recipient names resolve in the global address list).
If necessary, administrators can easily move mailboxes back to the on-premises Exchange environment.