using (ExchangeServiceBinding esb = new ExchangeServiceBinding())
{
    esb.Credentials = new NetworkCredential("ivanov", "pass", "firma");
    esb.Url = "http://mail-cas.firma.lan/ews/exchange.asmx";

    // Create a CreateItem request object
    CreateItemType request = new CreateItemType();

    // Setup the request:
    // Indicate that we only want to send the message. No copy will be saved.
    request.MessageDisposition = MessageDispositionType.SendOnly;
    request.MessageDispositionSpecified = true;

    // Create a message object and set its properties
    MessageType message = new MessageType();
    message.Subject = "TEST SOAP";
    message.Body = new BodyType();
    message.Body.BodyType1 = BodyTypeType.Text;
    message.Body.Value = "TEST SOAP";
    message.ToRecipients = new EmailAddressType[1];
    message.ToRecipients[0] = new EmailAddressType();
    message.ToRecipients[0].EmailAddress = "ivanov@firma.ru";//toEmailAddress;
    message.ToRecipients[0].RoutingType = "SMTP";

    //There are some more properties in MessageType object 
    //you can set all according to your requirement
    // Construct the array of items to send
    request.Items = new NonEmptyArrayOfAllItemsType();
    request.Items.Items = new ItemType[1];
    request.Items.Items[0] = message;

    // Call the CreateItem EWS method.
    CreateItemResponseType response = esb.CreateItem(request); // ОШИБКА
    //this.ThrowOnError("SendMessage", response.ResponseMessages.Items[0]);
}