The server rejected one or more recipient addresses. The server response was: 550 5.7.1 Unable to relay

i am trying to send an email confirmation to person who registered online after submit.   somehow it seems it only accepts authenticated users.   how do i fix this?

Server Error in '/WebSite1' Application.
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The server rejected one or more recipient addresses. The server response was: 550 5.7.1 Unable to relay for gianitoo@hotmail.com

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.Runtime.InteropServices.COMException: The server rejected one or more recipient addresses. The server response was: 550 5.7.1 Unable to relay for gianitoo@hotmail.com


Source Error:

The source code that generated this unhandled exception can only be shown when compiled in debug mode. To enable this, please follow one of the below steps, then request the URL:

1. Add a "Debug=true" directive at the top of the file that generated the error. Example:

  <%@ Page Language="C#" Debug="true" %>

or:

2) Add the following section to the configuration file of your application:

<configuration>
   <system.web>
       <compilation debug="true"/>
   </system.web>
</configuration>

Note that this second technique will cause all files within a given application to be compiled in debug mode. The first technique will cause only that particular file to be compiled in debug mode.

Important: Running applications in debug mode does incur a memory/performance overhead. You should make sure that an application has debugging disabled before deploying into production scenario.  

Stack Trace:

[COMException (0x8004020f): The server rejected one or more recipient addresses. The server response was: 550 5.7.1 Unable to relay for gianitoo@hotmail.com
]

[TargetInvocationException: Exception has been thrown by the target of an invocation.]
   System.RuntimeType.InvokeDispMethod(String name, BindingFlags invokeAttr, Object target, Object[] args, Boolean[] byrefModifiers, Int32 culture, String[] namedParameters) +0
   System.RuntimeType.InvokeMember(String name, BindingFlags bindingFlags, Binder binder, Object target, Object[] providedArgs, ParameterModifier[] modifiers, CultureInfo culture, String[] namedParams) +2501296
   System.Web.Mail.LateBoundAccessHelper.CallMethod(Object obj, String methodName, Object[] args) +72

[HttpException (0x80004005): The server rejected one or more recipient addresses. The server response was: 550 5.7.1 Unable to relay for gianitoo@hotmail.com
]
   System.Web.Mail.LateBoundAccessHelper.CallMethod(Object obj, String methodName, Object[] args) +119
   System.Web.Mail.CdoSysHelper.Send(MailMessage message) +2684
   System.Web.Mail.SmtpMail.Send(MailMessage message) +131
   _Default.EnterBtn_Click(Object sender, ImageClickEventArgs e) +170
   System.Web.UI.WebControls.ImageButton.OnClick(ImageClickEventArgs e) +105
   System.Web.UI.WebControls.ImageButton.RaisePostBackEvent(String eventArgument) +115
   System.Web.UI.WebControls.ImageButton.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +7
   System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +11
   System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +33
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +5102
 


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Version Information: Microsoft .NET Framework Version:2.0.50727.42; ASP.NET Version:2.0.50727.210