Troubleshooting "Mailbox unavailable. The server response was: Access denied - Invalid HELO name" when sending email with SmtpClient

Question

I have been trying to send an email by C#. I have Googled for various examples and have taken bits and pieces from each and from the standard code which everyone would most probably be using.

string to = "[email protected]";
string from = "[email protected]";
string subject = "Hello World!";
string body =  "Hello Body!";
MailMessage message = new MailMessage(from, to, subject, body);
SmtpClient client = new SmtpClient("smtp.domain.com");
client.Credentials = new NetworkCredential("[email protected]", "password");
client.Send(message);

However, I keep getting an error stating

System.Net.Mail.SmtpException: Mailbox unavailable. The server response was: Access denied - Invalid HELO name (See RFC2821 4.1.1.1)

So, what do I do now? Is SmtpClient supposed to be special and only work on specific SMTP servers?

Answer 1

It seems your username/password pair is not authenticating successfully with your SMTP server.

EDIT

I think, I found what's wrong here. I have corrected your version below.

string to = "[email protected]";

//It seems, your mail server demands to use the same email-id in SENDER as with which you're authenticating. 
//string from = "[email protected]";
string from = "[email protected]";

string subject = "Hello World!";
string body =  "Hello Body!";
MailMessage message = new MailMessage(from, to, subject, body);
SmtpClient client = new SmtpClient("smtp.domain.com");
client.UseDefaultCredentials = false;
client.Credentials = new NetworkCredential("[email protected]", "password");
client.Send(message);

Answer 2

Have you tried setting your auth credentials in the web.Config?

  <system.net>
    <mailSettings>
      <smtp from="[email protected]">
        <network host="smtpserver1" port="25" userName="username" password="secret" defaultCredentials="true" />
      </smtp>
    </mailSettings>
  </system.net>

and your code behind

MailMessage message = new MailMessage();
message.From = new MailAddress("[email protected]");
message.To.Add(new MailAddress("[email protected]"));
message.To.Add(new MailAddress("[email protected]"));
message.To.Add(new MailAddress("[email protected]"));
message.CC.Add(new MailAddress("[email protected]"));
message.Subject = "This is my subject";
message.Body = "This is the content";
SmtpClient client = new SmtpClient();
client.Send(message);