I tried to configure the mail server in activiti.cfg.xml directly in my project, but there is unfortunately no effect. If I configure the mail server in the activiti.cfg.xml in apache-tomcat-6.0.29\webapps\activiti-rest\WEB-INF\lib\activiti-cfg.jar\ then works. But it is very laborious.
The activiti.cfg.xml file in the rest-webapp is the engine configuration used for explorer and probe, so if you're using explorer and probe, this is the place to put the mail-server configuration. There is a servlet context-listener in the webapp that loads the ProcessEngine, based on the activiti.cfg.xml on the classpath.