We now recommend that you convert the on-premises mailboxes in the migration batch to mail-enabled users. Why? After an %BRAND_EXCHANGE_ONLINE_SHORT% ...

We now recommend that you convert the on-premises mailboxes in the migration batch to mail-enabled users. Why? After an %BRAND_EXCHANGE_ONLINE_SHORT% cutover migration, a user has an on-premises mailbox and an %BRAND_EXCHANGE_ONLINE_SHORT% Online mailbox in %BRAND_OFFICE_365_SHORT%. Because mail sent to the user's on-premises mailbox is sent directly to their %BRAND_EXCHANGE_ONLINE% mailbox after migration, users need to connect to their %BRAND_EXCHANGE_ONLINE% mailboxes to access their email. But if a person uses %BRAND_OUTLOOK_SHORT% to open their mailbox, the Autodiscover service will still attempt to connect to the on-premises mailbox. After you convert on-premises mailboxes to mail-enabled users, the Autodiscover service uses a mail-enabled user to connect %BRAND_OUTLOOK_SHORT% to the %BRAND_EXCHANGE_ONLINE_SHORT% Online mailbox after the user creates a new %BRAND_OUTLOOK_SHORT% profile.