I use multiple exchange accounts, how do i choose which account to use?
how do use oneplacemail with multiple exchange accounts
Do these accounts have Exchange online license? or are they shared mailboxes? OnePlaceMail App is not available for shared mailboxes.
if they are full mailboxes then you should see the app appearing in Outlook when you select the inbox. are you able to send some screenshots of the steps you follow to firstname.lastname@example.org so we can further assist you?