@ CHUAN,
Its possible… make us of WmPublic/pub.client:smtp service in your flow service (where you build logic) and follow the steps
1> expose the flow service (with ACL’s) as WS provider create WSD
2> Provide the WSDL URL/File to your partner/client
3> Test the above using SOAP UI to see whether you are able to invoke the WS that is residing on IS
To make it secure go for https
#Integration-Server-and-ESB#webMethods