Server Error in '/' Application.

I was making a purchase on line using K-Melon and when I went of connect to the payment page the following got thrown up. Can someone explain it please.

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Server Error in '/' Application.
A potentially dangerous Request.Form value was detected from the client (Email=" <********ilders@sta...").
Description: Request Validation has detected a potentially dangerous client input value, and processing of the request has been aborted. This value may indicate an attempt to compromise the security of your application, such as a cross-site scripting attack. You can disable request validation by setting validateRequest=false in the Page directive or in the configuration section. However, it is strongly recommended that your application explicitly check all inputs in this case.

Exception Details: System.Web.HttpRequestValidationException: A potentially dangerous Request.Form value was detected from the client (Email=" <********lders@sta...").

Source Error:

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.

Stack Trace:

[HttpRequestValidationException (0x80004005): A potentially dangerous Request.Form value was detected from the client (Email=" <********ders@sta...").]
System.Web.HttpRequest.ValidateString(String s, String valueName, String collectionName) +230
System.Web.HttpRequest.ValidateNameValueCollection(NameValueCollection nvc, String collectionName) +99
System.Web.HttpRequest.get_Form() +113
System.Web.UI.Page.GetCollectionBasedOnMethod() +69
System.Web.UI.Page.DeterminePostBackMode() +47
System.Web.UI.Page.ProcessRequestMain() +2106
System.Web.UI.Page.ProcessRequest() +218
System.Web.UI.Page.ProcessRequest(HttpContext context) +18
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication+IExecutionStep.Execute() +179
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +87


Version Information: Microsoft .NET Framework Version:1.1.4322.573; ASP.NET Version:1.1.4322.573
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