Espressif Systems ESP8266 Beginner's Manual page 51

Until there's support for the new OAuth2 protocol on the ESP, we'll have to use the old, less secure method.
Hardware
Connect an LED (+ resistor) to pin 13, as an unread email indicator.
The Code
#include <WiFiClientSecure.h> // Include the HTTPS library
#include <ESP8266WiFi.h> // Include the Wi-Fi library
#include <ESP8266WiFiMulti.h> // Include the Wi-Fi-Multi library
ESP8266WiFiMulti wifiMulti; // Create an instance of the ESP8266WiFiMulti class, called 'wifiMulti'
const char* host = "mail.google.com"; // the Gmail server
const char* url = "/mail/feed/atom"; // the Gmail feed url
const int httpsPort = 443; // the port to connect to the email server
// The SHA-1 fingerprint of the SSL certificate for the Gmail server (see below)
const char* fingerprint = "D3 90 FC 82 07 E6 0D C2 CE F9 9D 79 7F EC F6 E6 3E CB 8B B3";
// The Base64 encoded version of your Gmail login credentials (see below)
const char* credentials = "ZW1haWwuYWRkcmVzc0BnbWFpbC5jb206cGFzc3dvcmQ=";
const byte led = 13;
void setup() {
Serial.begin(115200); // Start the Serial communication to send messages to the computer
delay(10);
Serial.println('\n');
pinMode(led, OUTPUT);
wifiMulti.addAP("ssid_from_AP_1", "your_password_for_AP_1"); // add Wi-Fi networks you want to connect to
wifiMulti.addAP("ssid_from_AP_2", "your_password_for_AP_2");
wifiMulti.addAP("ssid_from_AP_3", "your_password_for_AP_3");
Serial.println("Connecting ...");
int i = 0;
while (wifiMulti.run() != WL_CONNECTED) { // Wait for the Wi-Fi to connect: scan for Wi-Fi networks, and connect to the strongest of
the networks above
delay(250);
Serial.print('.');
}
Serial.println('\n');
Serial.print("Connected to ");
Serial.println(WiFi.SSID()); // Tell us what network we're connected to
Serial.print("IP address:\t");
Serial.println(WiFi.localIP()); // Send the IP address of the ESP8266 to the computer
Serial.println('\n');
}
void loop() {
int unread = getUnread();
if (unread == 0) {
Serial.println("\r\nYou've got no unread emails");
digitalWrite(led, LOW);
} else if (unread > 0) {
Serial.printf("\r\nYou've got %d new messages\r\n", unread);
digitalWrite(led, HIGH);
} else {
Serial.println("Could not get unread mails");
}
Serial.println('\n');
delay(5000);
}
int getUnread() { // a function to get the number of unread emails in your Gmail inbox
WiFiClientSecure client; // Use WiFiClientSecure class to create TLS (HTTPS) connection
Serial.printf("Connecting to %s:%d ... \r\n", host, httpsPort);
if (!client.connect(host, httpsPort)) { // Connect to the Gmail server, on port 443
Serial.println("Connection failed"); // If the connection fails, stop and return
return -1;
}
if (client.verify(fingerprint, host)) { // Check the SHA-1 fingerprint of the SSL certificate
Serial.println("Certificate matches");