Mail transport is a polling based approach. And you need to setup a Proxy Service to poll your mails from a mail server. In you Proxy Service, you process your subject and body. Whether you need a custom Java class depends on how much you work with XML.
The more XML your message is, the less likely you need Java. If you are familiar with XQuery/XSLT, you can also in many ways avoid using custom Java to process your messages.
Once you have your messages ready for delivery, you can route them to a business service which may or may not be WSDL based.