Set custom header (x-header) on Outlook compose mail with JS addin

Question

I want to set a custom header in Outlook outgoing mail using an Outlook Web Add-in. How can we achieve this?

Answer 1

While Mark's (@MarkLaFleur) answer is correct and indeed the Office.js API doesn't provide any direct functionality to manipulate with transport layer headers, you are able to achieve this. There are two options available for you ...

• Office.context.mailbox.makeEwsRequestAsync() gives you ability to send EWS request to Exchange server and get/set properties of the item, including x-headers (which is just yet another property; refer to distinguished property "InternetHeaders", corresponding to Id {00020386-0000-0000-C000-000000000046}). Please see the example on github: Make an Exchange Web Service request from Outlook
• Make REST API call to restUrl with appropriate query to set/get the properties of the item. The example of such request you may find: Set a custom header with Outlook/Office 365 REST

UPDATE:

With Mailbox 1.8 API set new Office.InternetHeaders interface is available to set, get and remove Transport Headers of the message through regular JavaScript API calls.