You receive the following error message when attempting to sign in to an Outlook app:
You've reached your sign-in limit
To continue on this device, please sign out of Office on another device using your Microsoft account portal.
You have signed in to Outlook on more than five devices. With Microsoft 365, you can install Office on all your devices and sign in to Outlook on five devices simultaneously. This includes any combination of desktop computers, laptops, tablets, and phones.
Sign out of Outlook on another device.
You will be able to sign in on your preferred device.