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I’ve been trying to send email on a shared windows hosting from an Asp.NET page.  I finally found the solution for the problem.  It may be useful for those who have similar problems.

(I used System.Net.Mail namespace).

I was trying this code:

[csharp]
MailMessage msg = new MailMessage();
msg.From = new MailAddress("xxx@xxx.com");
msg.To.Add(new MailAddress("xxx@xxx.com"));
msg.Subject = "Test";
msg.Body = "Body Test";

SmtpClient client = new SmtpClient("smtp.xxx.com", 25);
client.Send(msg);
[/csharp]

This code gave this error:
“Mailbox unavailable. The server response was: 5.7.1″

When I was looking for the solution, I came up lots of suggestions.   Some of them required IIS configuration which I don’t have the chance to make.  Only one of them worked for me:
[csharp]
MailMessage msg = new MailMessage();
msg.From = new MailAddress("xxx@xxx.com");
msg.To.Add(new MailAddress("xxx@xxx.com"));
msg.Subject = "Test";
msg.Body = "Body Test";

SmtpClient client = new SmtpClient("smtp.xxx.com");
client.UseDefaultCredentials = false;
System.Net.NetworkCredential theCredential =
new System.Net.NetworkCredential("xxx@xxx.com", "xxx");
client.Credentials = theCredential;
client.Send(msg);
[/csharp]