Consider the following scenario. A user uses an "email as sign in" (EASI) account or a custom domain as his or her Microsoft account to sign in to Microsoft services. For example, the user uses firstname.lastname@example.org. The domain in the user's Microsoft account is the same as the domain that Microsoft Office 365 uses. In this scenario, the user tries to sign in to Skype. However, the user can't use his or her current Microsoft account to sign in to Skype. Instead, the user is forced to change his or her Microsoft account so that it uses a different domain.
To resolve this issue, use one of the following methods:
Method 1: For Office 365 Enterprise organizations, disable PIC in the Skype for Business (formerly Lync Online) Admin Center. To do this, follow the steps on the following Microsoft website:
Method 3: Use a different domain for your Microsoft account, or use a different domain in Office 365.
This issue occurs if the following conditions are true:
The user's Microsoft account uses the same domain name as Office 365.
Public IM connectivity (PIC) is enabled in Skype for Business Online for Office 365 Enterprise organizations.
External Communications is enabled in Skype for Business Online for Office 365 Small Business organizations.
In this scenario, when the user signs in to Skype, the user is forced to change his or her Microsoft account to use a domain that isn't in conflict with Office 365.
This issue occurs because of the way that the session initiation protocol (SIP) services are advertised. The Office 365 and Skype environments can coexist if PIC or External Communications is disabled for the Office 365 domain. However, if PIC or External Communications is enabled for the Office 365 domain, the Office 365 domain becomes the authoritative SIP domain and hosts the service. Therefore, when a user tries to sign in to Skype, the Microsoft account service recognizes that Office 365 controls the domain namespace and requires the user to change his or her Microsoft account to use a different domain.