you have dependency for workItemHandlers
<dependency>
<groupId>org.jbpm</groupId>
<artifactId>jbpm-workitems</artifactId>
<version>5.4.0.Final</version>
<exclusions>
<exclusion>
<groupId>dom4j</groupId>
<artifactId>dom4j</artifactId>
</exclusion>
</exclusions>
</dependency> ?
ksession.getWorkItemManager().registerWorkItemHandler("Human Task", humanTaskHandler);
ksession.getWorkItemManager().registerWorkItemHandler("Service Task", new org.jbpm.process.workitem.bpmn2.ServiceTaskHandler(ksession));
ksession.getWorkItemManager().registerWorkItemHandler("Email", new org.jbpm.process.workitem.email.EmailWorkItemHandler("smtp.yandex.ru","587","email@yandex.ru","password"));
ksession.getWorkItemManager().registerWorkItemHandler("Log", new org.jbpm.process.instance.impl.demo.SystemOutWorkItemHandler());
I try EmailWorkItemHandler, it work. Or you have custom emailhandler?