Enterprises use a multi-pronged strategy for email infrastructure due to both cost, manage-ability and scale reasons. Transactional emails (such as order confirmations, shipment notifications) or marketing emails are typically handled using 3rd party ESP. Even here, surveys show that more than 2/3rd of enterprises that use their home-grown email platform also use an external email service.
Sinch E-mail 365 does not support corporate emails. It services only outbound multi-channel notifications.
Corporate mail ecosystem are typically workgroup clusters consisting of Outlook/Exchange or Lotus Domino. They serve the purpose of establishing and managing individual or group email accounts servicing peer to peer communication using dedicated email clients. Sinch E-mail 365 delivers to recipient email clients; but is consumed by any enterprise application as a pure API interface.