This policy setting governs whether personal Microsoft accounts can be configured to use MAPI in Outlook.
A personal Microsoft account is an account hosted on Outlook.com, Hotmail.com, Live.com, Msn.com or any variation on those domains.
If you enable this policy, users won't be able to configure a personal Microsoft account to use MAPI in Outlook.
This means that in the Add Account dialog box in Outlook, users must choose "Manual setup or additional server types," then choose Next, and then choose "POP or IMAP."
If you disable or don't configure this policy setting, users can configure a personal Microsoft account to use MAPI in Outlook. They can do this by choosing "E-mail" account in the Add Account dialog box.
Note that personal Microsoft accounts that are configured to use MAPI will be automatically configured to use Cached Exchange Mode, and this can't be changed.