A message sent to adapter "HTTP" on send port "<SendPortName>" with URI "<URL>" is suspended.
Error details: The underlying connection was closed: An unexpected error occurred on a receive.
You can get the similar error for SOAP adapter too.
On production environment this error was occurring randomly once or twice in a month. After lot of investigation I came to know that the AppPool was getting recycled before Web Page can respond to the request.
Though there are no of reasons for AppPool recycle but in this case the IIS was hosted on a machine having sufficient resources(memory,CPU). I figured out that the app pool used for web application was configured to get recycled after 1740 minutes (which is default setting when you create new App Pool).
Open the Internet Information Services (IIS) Manager go to Application Pool and open the properties of the required application pool. Uncheck the "Recycle worker processes (in minutes):" check box.