web mail send problem

See more:

C#

ASP.NET

MailSlot

I want to send email from my asp.net application

I have successfully sent the email form my aspx page for .com domain

by the following code

MailMessage msg = new MailMessage();
        msg.From = new MailAddress("support@mydomain.com");
        msg.To.Add("recipiant@gmail.com");
        msg.Subject = "Demo Subject";
        msg.Body = "Hey Test email";        
        SmtpClient client = new SmtpClient();
        client.Host = "mail.mydomain.com";
        client.Port = 25;
        client.Credentials = new NetworkCredential("username", "password");        
        client.Send(msg);

But in case of .org domain it is not going email
I have used the below code for .org domain

MailMessage msg = new MailMessage();
msg.From = new MailAddress("support@mydomain.org");
msg.To.Add("recipiant@gmail.com");
msg.Subject = "Demo Subject";
msg.Body = "Hey Test email";
SmtpClient client = new SmtpClient();
client.Host = "mail.mydomain.org";
client.Port = 25;
client.Credentials = new NetworkCredential("username", "password");
client.Send(msg);

In user name filed i have entered only user name not with domain name.

so first sent when i have used for .com domain

but when i have used .org mail with correct id and password, then one error is throwing me, called:

Mailbox unavailable. The server response was: Authentication is required for relay

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Net.Mail.SmtpFailedRecipientException: Mailbox unavailable. The server response was: Authentication is required for relay