Not getting response from Gmail API (Users.messages: send) -- running on mobile device via IonicFramework + AngularJS

I am able to send emails successfully using GMail's API with no issues. The receiver gets the email after a few seconds, but the app doesn't get a response from GMail (i.e., threadid, id, etc...) described here.

I'm running this on a mobile device via IonicFramework w/c uses angularJS by the way. What could be wrong?

$http({
  method: 'POST',
  url: 'https://www.googleapis.com/upload/gmail/v1/users/me/messages/send?uploadType=multipart',
  headers: {
    'Authorization':'Bearer '+ access_token,
    'Content-Type': 'multipart/mixed; boundary="foo_bar_baz"'
  },
  data: emailStr
})
.success(function(data, status, headers, config){
  console.log(data);            
})
.error(function(data, status, headers, config){
   console.log(data);                
});

emailStr contains the base64-encoded message.

This is how the request details look like:

Request URL: https://www.googleapis.com/upload/gmail/v1/users/me/messages/send?uploadType=multipart
Request Headers
Accept:application/json, text/plain, */*
Authorization:Bearer <<token here>>
Content-Type:multipart/mixed; boundary="foo_bar_baz"
Origin:file://
User-Agent: <<User agent data>>
Query String Parameters
uploadType:multipart
Request Payload
--foo_bar_baz
Content-Type: application/json; charset="UTF-8"

{"raw":"<<encoded string>>"}
--foo_bar_baz--