Comment 60 for bug 215194

It's just a pipe on the input stream, see nsImapProtocol::SetupWithUrl. We create a transport on the io socket:
        rv = socketService->CreateTransport(&connectionType, connectionType != nsnull,
                                            *socketHost, socketPort, proxyInfo,
                                            getter_AddRefs(m_transport));

and then we open an input stream on that transport:
          rv = m_transport->OpenInputStream(nsITransport::OPEN_BLOCKING, 0, 0, getter_AddRefs(m_inputStream));