Hi Nikh,
According to your description, your organization has deployed single sign-on (SSO). In SSO environment, when federated users sign into Office 365 service, they will be redirected to on-premsie security token service (STS) infrastructure for authentication. When activating Outlook for mac, you will also be redirected to the STS infrastructure for authentication. For this kind of issue, based on our experience, it is hard to resolve it from client side. We need to involve the Office 365 admin to check SSO related settings.
Are you the Office 365 admin? If not, you can contact your admin about how your organization deploy SSO. To further analyze the issue and protect your privacy, I have sent a private message to request your configuration information (such as your identity provider, ADFS or 3rd party provider). To view the private message, please click Private messages on the right side of the community site -> Click the subject to view the message -> reply to the message. Please feel free to provide the information for further analysis.
The Microsoft officially supported scenario is to use ADFS to achieve SSO. If your organization is using any 3rd party SSO service/identity provider, the sign in authentication is controlled and managed by the 3rd party provider, and we cannot provide official support. You need to contact your Office 365 admin to resolve this issue. Thanks for your understanding and efforts.
Regards,
Neo
