Could be any number of things but from the looks of it you just don't have the network connectivity from the app to the mail server (proxy or pure connection issue).

+++ Rick ---

>My ASP.NET application that has been working without any problems for a long time started giving an error. The error seems to be starting with the application trying to send an email message. From the error message the custom sent me it looks like something to do with the System.Net.Sockets. Any idea of what might be causing this problem?
>
>Thank you. Below is a copy of the error message:
>
>

>[SocketException (0x2745): An established connection was aborted by the software
>in your host machine]
>   System.Net.Sockets.Socket.DoConnect(EndPoint endPointSnapshot, SocketAddress
>socketAddress) +1002146
>   System.Net.Sockets.Socket.InternalConnect(EndPoint remoteEP) +33
>   System.Net.ServicePoint.ConnectSocketInternal(Boolean connectFailure, Socket
>s4, Socket s6, Socket& socket, IPAddress& address, ConnectSocketState state,
>IAsyncResult asyncResult, Int32 timeout, Exception& exception) +431
>
>[WebException: Unable to connect to the remote server]
>   System.Net.ServicePoint.GetConnection(PooledStream PooledStream, Object
>owner, Boolean async, IPAddress& address, Socket& abortSocket, Socket&
>abortSocket6, Int32 timeout) +1447736
>   System.Net.PooledStream.Activate(Object owningObject, Boolean async, Int32
>timeout, GeneralAsyncDelegate asyncCallback) +190
>   System.Net.PooledStream.Activate(Object owningObject, GeneralAsyncDelegate
>asyncCallback) +21
>   System.Net.ConnectionPool.GetConnection(Object owningObject,
>GeneralAsyncDelegate asyncCallback, Int32 creationTimeout) +318
>   System.Net.Mail.SmtpConnection.GetConnection(String host, Int32 port) +227
>   System.Net.Mail.SmtpTransport.GetConnection(String host, Int32 port) +316
>   System.Net.Mail.SmtpClient.GetConnection() +42
>   System.Net.Mail.SmtpClient.Send(MailMessage message) +1485
>