...
The IS&T O365 representative will notify you at the pre-determined date and time when your Emails have been cut over. This means the email migration has completed and access to the old MIT Exchange Email has been cut off and access to your emails on O365 is online and active.
You will need to delete your old MIT Exchange from Apple Mail and add the new MIT O365 Mail account.
...
These were the screens that I saw after I deleted my old Exchange Email account from Apple Mail and clicked to set up a new Exchange Account in order to set up the MIT O365 Email.
...