The transport failed to connect to the server in mail sending process

Hi All,
we are sending the bulk mail using asp.net web application.
 
the mail server failed frequently in bulk mail sending.it will stopped after once in 1000 to 2000 mails. sometimes it will send without any error.
error message :
The transport failed to connect to the server
System.Web.HttpException: The transport failed to connect to the server.  
---> System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.
---> System.Runtime.InteropServices.COMException (0x80040213): The transport failed to connect to the server.      
--- End of inner exception stack trace ---     at System.RuntimeType.InvokeDispMethod(String name, BindingFlags invokeAttr, Object target, Object[] args, Boolean[] byrefModifiers, Int32 culture, String[] namedParameters)     at System.RuntimeType.InvokeMemb ...