ASP.NET

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    protected void Button1_Click(object sender, EventArgs e)<br />
    {<br />
        MailMessage message = new MailMessage("mailer@testdomain.com", "testaddress@gmail.com", "feedback", "HI!");<br />
        SmtpClient mailClient = new SmtpClient("mail.testdomain.com");<br />
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        mailClient.Port = 25;<br />
        mailClient.UseDefaultCredentials = false;<br />
        mailClient.Credentials = new NetworkCredential("mailer@testdomain.com", "test");<br />
        mailClient.DeliveryMethod = SmtpDeliveryMethod.Network;<br />
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        mailClient.Send(message);<br />
        <br />
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        message.Dispose();<br />
    }<br />

Server Error in '/' Application.<br />
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The SMTP server requires a secure connection or the client was not authenticated. The server response was: PLESK.home ESMTP MailEnable Service, Version: 1.95-- denied access at 02/18/07 02:02:23 <br />
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. <br />
<br />
Exception Details: System.Net.Mail.SmtpException: The SMTP server requires a secure connection or the client was not authenticated. The server response was: PLESK.home ESMTP MailEnable Service, Version: 1.95-- denied access at 02/18/07 02:02:23<br />
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Source Error: <br />
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An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.  <br />
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Stack Trace: <br />
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[SmtpException: The SMTP server requires a secure connection or the client was not authenticated. The server response was: PLESK.home ESMTP MailEnable Service, Version: 1.95-- denied access at 02/18/07 02:02:23]<br />
   System.Net.Mail.SmtpConnection.GetConnection(String host, Int32 port) +819256<br />
   System.Net.Mail.SmtpTransport.GetConnection(String host, Int32 port) +316<br />
   System.Net.Mail.SmtpClient.GetConnection() +42<br />
   System.Net.Mail.SmtpClient.Send(MailMessage message) +2004<br />
   Email.Button1_Click(Object sender, EventArgs e) +153<br />
   System.Web.UI.WebControls.Button.OnClick(EventArgs e) +105<br />
   System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) +107<br />
   System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +7<br />
   System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +11<br />
   System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +33<br />
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +5102<br />
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Version Information: Microsoft .NET Framework Version:2.0.50727.42; ASP.NET Version:2.0.50727.210