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Sending a Message in Exchange Online via REST from an Arduino MKR1000

This is part 2 of my MKR1000 article, in this previous post I looked at sending a Message via EWS using Basic Authentication.  In this Post I'll look at using the new Outlook REST API which requires using OAuth authentication to get an Access Token.

The prerequisites for this sketch are the same as in the other post with the addition of the ArduinoJson library which is used to parse the Authentication Results to extract the Access Token. Also the SSL certificates for the  and need to be uploaded to the devices using the wifi101 Firmware updater.

To use Token Authentication you need to register an Application in Azure with the Mail.Send permission. The application should be a Native Client app that use the Out of Band Callback urn:ietf:wg:oauth:2.0:oob. You need to authorize it in you tenant (eg build a small app that can do that which will prompt for authorization). One that is done you then need to set that ClientId variable in the sketch

String ClientId = "8fe353d6-efa0-4b0f-aafb-ab7cf3a9b307";

I've put a copy of this Sketch up the code looks like

#include <ArduinoJson.h>

#include <Base64.h>
#include <ArduinoHttpClient.h>

This example creates a client object that connects and transfers
data using always SSL.

It is compatible with the methods normally related to plain
connections, like client.connect(host, port).

Written by Arturo Guadalupi
last revision November 2015


#include <SPI.h>
#include <WiFi101.h>

char ssid[] = "SSOecure"; //  your network SSID (name)
char pass[] = "pass@#";    // your network password (use for WPA, or use as key for WEP)
int keyIndex = 0;            // your network key Index number (needed only for WEP)
const size_t MAX_CONTENT_SIZE = 5120;

//Office365 Credentials
String ExUserName = "";
String ExPassword = "passw";
String ClientId = "8fe353d6-efa0-4b0f-aafb-ab7cf3a9b307";
//Message Details
String Auth = ExUserName + ":" + ExPassword;
String Subject = "Subject of the Message";
String To = "";
String Body = "Something happening in the Body";
String Access_Token = "";

bool DebugResponse = false;

int cCount = 0;

int status = WL_IDLE_STATUS;
// if you don't want to use DNS (and reduce your sketch size)
// use the numeric IP instead of the name for the server:

// Initialize the Ethernet client library
// with the IP address and port of the server
// that you want to connect to (port 80 is default for HTTP):
WiFiSSLClient client;

void setup() {
  //Initialize serial and wait for port to open:
  while (!Serial) {
    ; // wait for serial port to connect. Needed for native USB port only

  // check for the presence of the shield:
  if (WiFi.status() == WL_NO_SHIELD) {
    Serial.println("WiFi shield not present");
    // don't continue:
    while (true);

  // attempt to connect to Wifi network:
  while (status != WL_CONNECTED) {
    Serial.print("Attempting to connect to SSID: ");
    // Connect to WPA/WPA2 network. Change this line if using open or WEP network:
    status = WiFi.begin(ssid, pass);

    // wait 10 seconds for connection:
  Serial.println("Connected to wifi");
  if(Access_Token.length() > 0){
      Serial.println("Send Message");


void TokenAuth(String ClientId,String UserName, String Password)
     char endOfHeaders[] = "\r\n\r\n";
     char passwordCA[Password.length()+1];
     String content = "" + ClientId + "&grant_type=password&username=" + ExUserName + "&password=" + URLEncode(passwordCA) + "&scope=openid";
     Serial.println("\nStarting connection to server...");
     if (client.connectSSL("", 443)) {
       Serial.println("connected to server");
       client.print("POST ");
       client.println(" HTTP/1.1");
       client.println("Content-Type: application/x-www-form-urlencoded");
       client.println("client-request-id: " + ClientId);
       client.println("return-client-request-id: true");
       client.println("x-client-CPU: x32");
       client.println("x-client-OS: Arduino");
       client.print("Content-Length: ");
       client.println("Expect: 100-continue");
       bool ok =  client.find(endOfHeaders);
       if (!ok) {
         Serial.println("No response or invalid response!");
         Serial.println("Request Okay");
       char response[MAX_CONTENT_SIZE];
       readAuthReponse(response, sizeof(response));

void readAuthReponse(char* content, size_t maxSize) {
  size_t length = client.readBytes(content, maxSize);
  content[length] = 0;
  content[length] = 0;
  DynamicJsonBuffer jsonBuffer;
  JsonObject&root = jsonBuffer.parseObject(content);
  if (!root.success()) {
    Serial.println("JSON parsing failed!");
      String token = root["access_token"]; 
      Access_Token = token;

String URLEncode(const char* msg)
    const char *hex = "0123456789abcdef";
    String encodedMsg = "";

    while (*msg!='\0'){
        if( ('a' <= *msg && *msg <= 'z')
                || ('A' <= *msg && *msg <= 'Z')
                || ('0' <= *msg && *msg <= '9') ) {
            encodedMsg += *msg;
        } else {
            encodedMsg += '%';
            encodedMsg += hex[*msg >> 4];
            encodedMsg += hex[*msg & 15];
    return encodedMsg;

void SendRest(String Bearer,String MessageTo, String MessageSubject, String MessageBody)
    //DebugResponse = true;
    char endOfHeaders[] = "\r\n\r\n";  
    String content = "{";
    content += "        \"Message\": {";  
    content += "           \"Subject\": \"" + MessageSubject + "\",";  
    content += "            \"Body\": {";  
    content += "                \"ContentType\": \"Text\",";  
    content += "                \"Content\": \"" + MessageBody + "\"";  
    content += "                       },";  
    content += "            \"ToRecipients\": [";  
    content += "                {";  
    content += "                    \"EmailAddress\": {";  
    content += "                        \"Address\": \"" + MessageTo + "\"";  
    content += "                    }";  
    content += "                }";  
    content += "            ]";  
    content += "        },";  
    content += "        \"SaveToSentItems\": \"false\"";  
    content += "    }";  
    Serial.println("\nStarting connection to server...");
    // if you get a connection, report back via serial:
    if (client.connectSSL("", 443)) {
      Serial.println("connected to server");
      client.print("POST ");
      client.println(" HTTP/1.1"); 
      client.print("Host: "); 
      client.print("Authorization: Bearer ");
      client.println("Connection: close");
      client.print("Content-Type: ");
      client.println("User-Agent: mrk1000Sender");
      client.print("Content-Length: ");
      char okayString[] = "HTTP/1.1 202 Accepted";
      bool ok =  client.find(okayString);
      if (!ok) {
        Serial.println("Message Sent");
        Serial.println("Request Failed");


void loop() {
  // if there are incoming bytes available
  // from the server, read them and print them:
  while (client.available()) {
    char c =;

  // if the server's disconnected, stop the client:
  if (!client.connected()) {
    Serial.println("disconnecting from server.");
    // do nothing forevermore:
    while (true);

void printWifiStatus() {
  // print the SSID of the network you're attached to:
  Serial.print("SSID: ");

  // print your WiFi shield's IP address:
  IPAddress ip = WiFi.localIP();
  Serial.print("IP Address: ");

  // print the received signal strength:
  long rssi = WiFi.RSSI();
  Serial.print("signal strength (RSSI):");
  Serial.println(" dBm");

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All sample scripts and source code is provided by for illustrative purposes only. All examples are untested in different environments and therefore, I cannot guarantee or imply reliability, serviceability, or function of these programs.

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