One of our customers removed their ISA proxy and had removed all the proxy settings from all the send ports but one set of WCF-Custom send adapters showed an erratic behaviour.
For short periods of time the request returned successful responses but at other times we received this response
System.ServiceModel.EndpointNotFoundException: There was no endpoint listening at https://someservice that could accept the message. This is often caused by an incorrect address or SOAP action. See InnerException, if present, for more details. —> System.Net.WebException: Unable to connect to the remote server —> System.Net.Sockets.SocketException: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond …
This error only disappeared from the logs when the useDefaultWebProxy setting on httpsTransport behaviour was changed from True to False on all of the affected send ports.