How to retrieve callback token for sending emails for API set v1.3 - outlook-addin

Question

Scenario:
Permission on the Manifest: ReadWriteMailbox
We have a outlook add-in that has a button that sends email from the compose mode of OWA/Outlook Desktop add-in. We are doing that by
Get the callback token.
Get item rest id.
Get rest url (which turns out to be https://outlook.office365.com/api).
Making a POST call to https://resturl/v2.0/me/messages/restId/send
dataType: 'json',
Headers
'Authorization': 'Bearer callbackToken'
Problem:
The email sends successfully in OWA (Outlook Web) but on Outlook 2016 Desktop we get an 403 Forbidden response with
code: ErrorAccessDenied
message: The api you are trying to access does not support item scoped OAuth.
Our Analysis
As far as we have investigated, Outlook 2016 MSI install does not support v1.5 of API requirement set
see this:
API requirement set
Since, there are two getCallbackTokenAsync methods as per
https://dev.office.com/reference/add-ins/outlook/1.5/Office.context.mailbox?product=outlook&version=v1.5
we are using the one with the isRest option set to true, i.e. the v1.5 one
Our code to get the callback token:
Office.context.mailbox.getCallbackTokenAsync({
isRest: true
},
function(asyncResult) {
if (asyncResult.status === "succeeded") {
deferred.resolve(asyncResult.value);
} else {
deferred.reject("Could not retrieve token");
}
});
Code to get rest id
function _getItemRestId() {
// Currently the only Outlook Mobile version that supports add-ins
// is Outlook for iOS.
if (Office.context.mailbox.diagnostics.hostName === 'OutlookIOS') {
// itemId is already REST-formatted
return Office.context.mailbox.item.itemId || pvt.currentMailId;
} else {
// Convert to an item ID for API v2.0
return Office.context.mailbox.convertToRestId(Office.context.mailbox.item.itemId || pvt.currentMailId, Office.MailboxEnums.RestVersion.v2_0);
}
}
Code to get the rest url
function _getRestUrl() {
$timeout(function() {
notificationService.showInfo('This is the rest URL==> ' + Office.context.mailbox.restUrl);
});
console.log('Office REST URL =>', Office.context.mailbox.restUrl);
return Office.context.mailbox.restUrl || 'https://outlook.office.com/api'; // This is because this requires v1.5 api set requirements which is not yet supported in Outlook for Windows.
}
Most Probably,
the getCallbackTokenAsync that executes on Outlook Desktop is that of v1.3 API set and gets us some different token (for getting attachments for an item i guess.) although I am not 100% sure that this is exactly what is wrong with our app.
Will be grateful if I can get a fix to send emails using API v1.4 requirement set.
There is nothing on other forums regarding this and also on the official documentation for that matter.