Why this small change is important from an extensibility point of view is that when you save an Item this way, it creates a draft in the Mailbox you activated the compose app from which you can then modify anyway you like using the EWS updateItem operation. (normally you would be restricted in a compose app to just using the subset of properties and methods provided for in the Office.js).
One trick however is the ItemId which you need to make an UpdateItem EWS Request to update the Draft Item is still only available in Read Mode. So to get around this I've come up with the following workaround
- Set a custom property on the draft Item using the Mail App Custom properties method using something like https://msdn.microsoft.com/EN-US/library/office/fp142263.aspx . With my Mail Apps I generate a new GUID for the app session that I use as the value.
- Save the Item your composing as a Draft using saveAync
- Now you can use makeEwsRequestAsync to do a findItems on the drafts folder of the Mailbox with a restriction so it only finds the Item with the Custom property you have set. The Custom properties you can set in the Mail App API are documented in https://msdn.microsoft.com/en-us/library/hh968549(v=exchg.80).aspx . But basically what you have is they are a named property within the PS_PUBLIC_STRING propset of type String. The property name of the property is prefixed with cecp- and rest of the property name is the GUID of your Mail Apps as defined in the application's manifest Id .
- Once you have the ItemId and Change key you can then use UpdateItem in EWS to update any of the extended or strongly type properties on the draft Message.
Also because this is using version 1.3 of the Mail App Api you should set the requirements in the Manifest file of your application eg
This will mean your Mail App will only activate in clients that support version 1.3 which means at the moment it won't activate in Outlook 2013.