Came across this problem a little while ago, we had deployed a production system where we had 2 BizTalk servers which were sending messages to a large .net application. There were 9 web servers hosting the .net web services.
Intermittently we were getting the following error message taken from the event log on the BizTalk server.
Event Type: Warning
Event Source: BizTalk Server 2006
Event Category: BizTalk Server 2006
Event ID: 5743
The adapter failed to transmit message going to send port "Snd_AcmeApplicationService" with URL "wse://http://<Removed>/Acme.WebServices/CustomWebService.asmx". It will be retransmitted after the retry interval specified for this Send Port. Details:"The underlying connection was closed: An unexpected error occurred on a receive.".
Upon investigation the network guys found that 2 of the 9 web servers had been configured with the wrong Subnet Mask and when the request hit either of these boxes it was failing and the other boxes were fine.