The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required

When  you are getting the below error.

SMTP server requires a secure connection or the client was not authenticated. The server response was : 5.5.1 Authentication Required.

Then you need to configure your email ID using Microsoft Dynamics CRM Email Router Configuration Manager.

Download Email Router Configuration Manager according to your system configuration

And Install it in your system, then configure your email( from which mail you want to send mails in code).

When you install the Microsoft Dynamics CRM Email Router Configuration Manager,

Click on

Then click on Configuration files-àNewà then you will get the following screen.

Under Access Credentials you need to specify email id and password( which email you want to configure) then click on publish then write the following code.

Email sending in C# gmail

using (MailMessage mm = new MailMessage("from@gmail.com", "to@gmail.com"))

            {

                mm.Subject = "Subject Text";

                mm.Body = "Body Text";

                mm.IsBodyHtml = false;

                SmtpClient smtp = new SmtpClient();

                smtp.Host = "smtp.gmail.com";

                smtp.EnableSsl = true;

                NetworkCredential NetworkCred = new NetworkCredential("from@gmail.com", "pwm");

                ServicePointManager.ServerCertificateValidationCallback =

                delegate(object s, X509Certificate certificate,

                         X509Chain chain, SslPolicyErrors sslPolicyErrors)

                { return true; };

                smtp.UseDefaultCredentials = false;

                smtp.Credentials = NetworkCred;

                smtp.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network;

                smtp.Port = 587;

                smtp.Send(mm);

                //ClientScript.RegisterStartupScript(GetType(), "alert", "alert('Email sent.');", true);

            }