Demonstrates how to send and receive emails with OAuth2 authentication and Office 365

OAuth2_O365.cs

namespace OAuthWithO365
{
    using System;
    using System.Collections.Generic;
    using System.Net.Http;
    using System.Threading.Tasks;

    using MailKit;
    using MailKit.Net.Imap;
    using MailKit.Net.Smtp;
    using MailKit.Search;
    using MailKit.Security;
    using MimeKit;
    using Newtonsoft.Json.Linq;

    class Program {
        static readonly string tenant = "some GUID";
        static readonly string smtp_server = "smtp.office365.com";
        static readonly string imap_server = "outlook.office365.com";
        static readonly string aad_app_id = "some GUID";
        static readonly string aad_app_secret = "***** ***";
        static readonly string user = "username@domain";
        static readonly string pass = "***** ***";

        /// <summary>
        /// W oparciu o aplikację OAuth zarejestrowaną w AAD z uprawnieniami User.Read, Mail.Send
        /// generuje token umożliwiający wysyłanie maili w imieniu uzytkownika <see cref="mailUser"/>
        /// https://docs.microsoft.com/en-us/azure/active-directory/develop/v2-oauth-ropc
        /// </summary>
        /// <returns>Access token owinięty w specyficzna dla MailKit strukture</returns>
        static async Task<SaslMechanismOAuth2> GetAccessToken(string api_perm)
        {
            var content = new FormUrlEncodedContent(new List<KeyValuePair<string, string>>
            {
                    new KeyValuePair<string, string>("scope", api_perm),
                    new KeyValuePair<string, string>("grant_type", "password"),
                    new KeyValuePair<string, string>("resource", "https://outlook.office365.com"),
                    new KeyValuePair<string, string>("client_id", aad_app_id),
                    new KeyValuePair<string, string>("client_secret", aad_app_secret),
                    new KeyValuePair<string, string>("username", user),
                    new KeyValuePair<string, string>("password", pass)
                 });

            var client = new HttpClient();
            var response = await client.PostAsync($"https://login.microsoftonline.com/{tenant}/oauth2/token", content).ConfigureAwait(continueOnCapturedContext: false);
            var jsonString = await response.Content.ReadAsStringAsync();
            var jobj = JObject.Parse(jsonString);
            var token = jobj["access_token"];
            client.Dispose();

            return new SaslMechanismOAuth2(user, token.ToString());
        }

        static void SendTo365()
        {
            var at = GetAccessToken("SMTP.Send").Result;
            using (var client = new SmtpClient(/*new MailKit.ProtocolLogger("smtp.log")*/)) {
                client.ServerCertificateValidationCallback = (s, c, h, e) => true;

                try {
                    client.Connect(smtp_server, 587, SecureSocketOptions.Auto);
                    client.Authenticate(at);
                    var msg = new MimeMessage();
                    msg.From.Add(MailboxAddress.Parse(user));
                    msg.To.Add(MailboxAddress.Parse(user));
                    msg.Subject = "Testing SMTP";
                    msg.Body = new TextPart("plain"){ Text = "Zażółć gęślą jaźń\n" + DateTime.Now.ToLongTimeString() };
                    client.Send(msg);
                } catch (Exception ex) {
                    Console.WriteLine(ex.Message);
                }
            }
        }

        static void ReceiveFrom365()
        {
            var at = GetAccessToken("IMAP.AccessAsUser.All").Result;
            using (var client = new ImapClient(/*new MailKit.ProtocolLogger("imap.log")*/)) {
                client.ServerCertificateValidationCallback = (s, c, h, e) => true;

                try {
                    client.Connect(imap_server, 993, SecureSocketOptions.Auto);
                    client.Authenticate(at);

                    client.Inbox.Open(FolderAccess.ReadOnly);
                    var uids = client.Inbox.Search(SearchQuery.New);
                    foreach (var uid in uids) {
                        var msg = client.Inbox.GetMessage(uid);
                        Console.WriteLine("\nGot email from: " + msg.From[0].ToString());
                        Console.WriteLine(msg.TextBody);
                    }
                } catch (Exception ex) {
                    Console.WriteLine(ex.Message);
                }
            }
        }

        static void Main(string[] args)
        {
            SendTo365();
            System.Threading.Thread.Sleep(2000);
            ReceiveFrom365();
        }
    }
}