it's working! I've got a simple mail application running now and like you said, there shouldn't anything more block me from integrating this in more sophisticated workflows. thanks everybody :)
in case someone is facing a similar problem, this is the code snippet I used (in ProcessMain.java):
// load up the knowledge base
KnowledgeBase kbase = readKnowledgeBase();
StatefulKnowledgeSession ksession = kbase.newStatefulKnowledgeSession();
EmailWorkItemHandler emailHandler = new EmailWorkItemHandler();
emailHandler.setConnection("smtp.gmail.com", "587", "<username>@gmail.com", "<password>");
emailHandler.getConnection().setStartTls(true);
ksession.getWorkItemManager().registerWorkItemHandler("Email", emailHandler);
// start a new process instance
ksession.startProcess("com.sample.bpmn.hello");