The present invention provides Web Services over a mail transport. In particular, the present invention sends Web Service messages using MAPI (Messaging Application Program Interface) mail applications in order to support offline communications and extended processing time of service requests without creating additional infrastructure for such message exchanges. A custom layer transport is configured to: identify that a Web Service message is to be sent across a mail transport; use the MAPI application to generate a mail message; and include the Web Service message and other identifiers within the mail message. The message is then sent to the outgoing mailbox of the MAPI application for subsequent delivery to an endpoint. Upon receiving the mail message, the endpoint can extract the Web Service message and process it accordingly. Other example embodiments also provide for requesting NPDL (Network Protocol Description Language) data structures (e.g., WSDL documents) by a similar process.