I don't want to open a new thread, so I'm posting this here. Since yesterday, I get several error messages after I have clicked on the submit button. Like this one:

Server Error in '/' Application.
Syntax error, command unrecognized. The server response was: 5.7.0 Poor mailing list quality
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.SmtpException: Syntax error, command unrecognized. The server response was: 5.7.0 Poor mailing list quality

Source Error:


Line 35:
Line 36: SmtpClient mailClient = new SmtpClient();
Line 37: mailClient.Send(message);
Line 38:
Line 39: }


Source File: c:\Websites\mydeathspace.com\submission.aspx.cs Line: 37

Stack Trace:


[SmtpException: Syntax error, command unrecognized. The server response was: 5.7.0 Poor mailing list quality]
System.Net.Mail.RecipientCommand.CheckResponse(Smt pStatusCode statusCode, String response) +1930951
System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, SmtpFailedRecipientException& exception) +346
System.Net.Mail.SmtpClient.Send(MailMessage message) +1957494
submission.btnSubmit_Click(Object sender, EventArgs e) in c:\Websites\mydeathspace.com\submission.aspx.cs:37
System.Web.UI.WebControls.Button.OnClick(EventArgs e) +115
System.Web.UI.WebControls.Button.RaisePostBackEven t(String eventArgument) +140
System.Web.UI.Page.RaisePostBackEvent(IPostBackEve ntHandler sourceControl, String eventArgument) +29
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2981
What's wroooooooooooooooooooooooong?