Outlook needs a service that conforms to the Office Mobile Service (OMS) guidelines in order to send and receive text messages. The OMS guidelines describe how to use a basic SOAP web service to expose standard endpoints for Outlook to discover service and user information and send text messages. Outlook calls each method it passes in XML data, which your service can parse to determine who is sending the message, what the body of the message is, and who the message is being sent to. To receive incoming messages, you need to create an endpoint that Twilio can call when it receives a message, and then forward it via email into Outlook, using specific headers in the mail message to indicate that it originated as an SMS message. A full sample on how to create an OMS service is available, which includes sending messages, handling incoming messages, and a simple website for users to provide their own Twilio credentials.