As you are Transitioned from BPOS-S into Office 365, you may have users/admins who continue to use the BPOS-S Microsoft Online Admin Center (MOAC), which will redirect all users into the Microsoft Online Portal (MOP). This is done via the BPOS Service determining the BPOS tenants has been Transitioned and issuing an http “meta refresh” redirect to https://portal.microsoftonline.com for logins. For OWA “redirects”, since a users BPOS-S MBX is moved into Exchange Online 365, the BPOS user’s targetAddress is set to Exchange Online 365. This means that OWA and Outlook will use this address to chase down and find their newly moved MBX.
How long will these redirects last?
As long as users still have a valid login and password, they can login to BPOS, either MOAC for Admins or Home/Company Portal for end-users and receive the redirect. Once the user’s password expires, the redirects will stop working. This is due to the user going through the “Change Password” operation, and will then receive a “Page Not Found”, as the tenant has been Transitioned.
So users can use the BPOS-S to Office 365 redirect as long as their BPOS credentials are valid and usable. Once the users password has expired, redirects will not work any longer!