ExchangeVersion requestedServerVersion = ExchangeVersion.Exchange2013_SP1;
  ExchangeService esb = new ExchangeService(requestedServerVersion);
  esb.Url = new Uri("https://mail.наш-holding.com/EWS/Exchange.asmx");
  esb.UseDefaultCredentials = true;

  // Формируем сообщение
  EmailMessage mail = new EmailMessage(esb);

  mail.Subject = "subject";
  mail.Body = new MessageBody(BodyType.Text, "body");
  mail.IsReadReceiptRequested = false;
  mail.ToRecipients.AddRange("тут какой-то получатель");
  mail.Send();

  esb.Url = new Uri("https://outlook.office365.com/EWS/Exchange.asmx");
  esb.UseDefaultCredentials = false;
  esb.Credentials = new WebCredentials(user, password, domain);