Use the Outlook REST APIs from an Outlook add-in

The Office.context.mailbox.item namespace provides access to many of the common fields of messages and appointments. However, in some scenarios an add-in may need to access data that isn't exposed by the namespace. For example, the add-in may rely on custom properties set by an outside app, or it needs to search the user's mailbox for messages from the same sender. In these scenarios, the Outlook REST APIs is the recommended method to retrieve the information.

Get an access token

The Outlook REST APIs require a bearer token in the Authorization header. Typically apps use OAuth2 flows to retrieve a token. However, add-ins can retrieve a token without implementing OAuth2 by using the new Office.context.mailbox.getCallbackTokenAsync method introduced in the Mailbox requirement set 1.5.

By setting the isRest option to true, you can request a token compatible with the REST APIs.

Add-in permissions and token scope

It is important to consider what level of access your add-in will need via the REST APIs. In most cases, the token returned by getCallbackTokenAsync will provide read-only access to the current item only. This is true even if your add-in specifies the read/write item permission level in its manifest.

If your add-in will require write access to the current item or other items in the user's mailbox, your add-in must specify the read/write mailbox permission. level in its manifest. In this case, the token returned will contain read/write access to the user's messages, events, and contacts.

Example

Office.context.mailbox.getCallbackTokenAsync({isRest: true}, function(result){
  if (result.status === "succeeded") {
    const accessToken = result.value;

    // Use the access token.
    getCurrentItem(accessToken);
  } else {
    // Handle the error.
  }
});

Get the item ID

To retrieve the current item via REST, your add-in will need the item's ID, properly formatted for REST. This is obtained from the Office.context.mailbox.item.itemId property, but some checks should be made to ensure that it is a REST-formatted ID.

• In Outlook on mobile devices, the value returned by Office.context.mailbox.item.itemId is a REST-formatted ID and can be used as-is.
• In other Outlook clients, the value returned by Office.context.mailbox.item.itemId is an EWS-formatted ID, and must be converted using the Office.context.mailbox.convertToRestId method.
• Note you must also convert Attachment ID to a REST-formatted ID in order to use it. The reason the IDs must be converted is that EWS IDs can contain non-URL safe values which will cause problems for REST.

Your add-in can determine which Outlook client it is loaded in by checking the Office.context.mailbox.diagnostics.hostName property.

Example

function getItemRestId() {
  if (Office.context.mailbox.diagnostics.hostName === 'OutlookIOS') {
    // itemId is already REST-formatted.
    return Office.context.mailbox.item.itemId;
  } else {
    // Convert to an item ID for API v2.0.
    return Office.context.mailbox.convertToRestId(
      Office.context.mailbox.item.itemId,
      Office.MailboxEnums.RestVersion.v2_0
    );
  }
}

Get the REST API URL

The final piece of information your add-in needs to call the REST API is the hostname it should use to send API requests. This information is in the Office.context.mailbox.restUrl property.

Example

// Example: https://outlook.office.com
const restHost = Office.context.mailbox.restUrl;

Call the API

After your add-in has the access token, item ID, and REST API URL, it can either pass that information to a back-end service which calls the REST API, or it can call it directly using AJAX. The following example calls the Outlook Mail REST API to get the current message.

function getCurrentItem(accessToken) {
  // Get the item's REST ID.
  const itemId = getItemRestId();

  // Construct the REST URL to the current item.
  // Details for formatting the URL can be found at
  // https://zcusa.951200.xyz/previous-versions/office/office-365-api/api/version-2.0/mail-rest-operations#get-messages.
  const getMessageUrl = Office.context.mailbox.restUrl +
    '/v2.0/me/messages/' + itemId;

  $.ajax({
    url: getMessageUrl,
    dataType: 'json',
    headers: { 'Authorization': 'Bearer ' + accessToken }
  }).done(function(item){
    // Message is passed in `item`.
    const subject = item.Subject;
    ...
  }).fail(function(error){
    // Handle error.
  });
}

See also

• For an example that calls the REST APIs from an Outlook add-in, see command-demo on GitHub.
• Outlook REST APIs are also available through the Microsoft Graph endpoint but there are some key differences, including how your add-in gets an access token. For more information, see Outlook REST API via Microsoft Graph.