Paypal Payment Gateway Integration in asp.net

[IOException: Authentication failed because the remote party has closed the transport stream.]
   System.Net.Security.SslState.StartReadFrame(Byte[] buffer, Int32 readBytes, AsyncProtocolRequest asyncRequest) +1100890
   System.Net.Security.SslState.StartReceiveBlob(Byte[] buffer, AsyncProtocolRequest asyncRequest) +178
   System.Net.Security.SslState.CheckCompletionBeforeNextReceive(ProtocolToken message, AsyncProtocolRequest asyncRequest) +107
   System.Net.Security.SslState.StartSendBlob(Byte[] incoming, Int32 count, AsyncProtocolRequest asyncRequest) +248
   System.Net.Security.SslState.ForceAuthentication(Boolean receiveFirst, Byte[] buffer, AsyncProtocolRequest asyncRequest) +130
   System.Net.Security.SslState.ProcessAuthentication(LazyAsyncResult lazyResult) +85
   System.Net.TlsStream.CallProcessAuthentication(Object state) +47
   System.Threading.ExecutionContext.runTryCode(Object userData) +66
   System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData) +0
   System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state) +166
   System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) +145
   System.Net.TlsStream.ProcessAuthentication(LazyAsyncResult result) +728
   System.Net.TlsStream.Write(Byte[] buffer, Int32 offset, Int32 size) +44
   System.Net.PooledStream.Write(Byte[] buffer, Int32 offset, Int32 size) +25
   System.Net.ConnectStream.WriteHeaders(Boolean async) +122
[WebException: The underlying connection was closed: An unexpected error occurred on a send.]
   System.Web.Services.Protocols.WebClientProtocol.GetWebResponse(WebRequest request) +314
   System.Web.Services.Protocols.HttpWebClientProtocol.GetWebResponse(WebRequest request) +4
   System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object[] parameters) +172
   SNPPayPal.com.paypal.sandbox.www.PayPalAPIAASoapBinding.SetExpressCheckout(SetExpressCheckoutReq SetExpressCheckoutReq) in D:\web\karthik\SNPPayPal\SNPPayPal\Web References\com.paypal.sandbox.www\Reference.cs:1453
   SNPPayPal.CheckOutExpress.Button1_Click(Object sender, EventArgs e) in D:\web\karthik\SNPPayPal\SNPPayPal\CheckOutExpress.aspx.cs:50
   System.Web.UI.WebControls.Button.OnClick(EventArgs e) +105
   System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) +107
   System.Web.UI.WebControls.Button.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) +1746