How to tell Rabbit to that task fails in .Net Core

Let's imagine that we have Q named "NotificationQ" and have a consumer who gets a task from that Q and sends emails to customers.

Emailing process sends an email by API from mailgun. That API request does not turn 200 every time(the reason is not important). In that time I need to tell RabbitMQ that tasks fail. I know there is a feature called autoAck but if a request fails how the RabbitMQ client pack understood that a fail.

Am I manually trigger ack to say that request failed?

I using https://www.nuget.org/packages/RabbitMQ.Client/ pack to handle RabbitMQ tasks.

            var channel = RabbitPrepareFactory.GetConnectionFactory();

            channel.BasicQos(0, 1, false);

            var notificationPack = channel.BasicGet("notification", true);


            var message = System.Text.Encoding.UTF8.GetString(notificationPack.Body.ToArray());


            var task = JsonConvert.DeserializeObject<ForgetPasswordEmailNotification>(message);


            var isEmailSendSuccessful = SomeFakeEmailSendFunctions(task.Email);


            if (!isEmailSendSuccessful)
            {
                //something for tell RabbitMQ that task fail and not delte that task in q
                .......
            }