The purpose of this paper is to propose an asynchronous service-oriented approach for B2B applications integration for web services. Currently, most web services are synchronous since their interaction protocol SOAP is based upon a synchronous transport protocol HTTP. However, synchronous web services presents limitations for integrating applications across businesses if one of the applications fails or the application needs longer interaction time due to its business requirements. For the asynchronous service-oriented approach, an asynchronous web service message handling middleware using existing email servers and .NET web services is developed since the existing email servers are ubiquitous and have a natural queuing mechanism and an asynchronous mechanism for .NET web services are not available contrary to the open source web service engine, AXIS. Creating such a middleware allows service providers to deploy asynchronous web services with as much easy as synchronous web services.