I just recognized an error in my mail-session configuration. The configuration section should be "swarm.mail.mail-session" - not "mail-session" (indention error). I further failed with the sessions jndi-name which has to be "java:jboss/mail/whatever". I'll update the example asap^^
When configured correctly you should get the following output on application start:
2018-08-06 15:17:29,410 INFO [org.jboss.as.mail.extension] (MSC service thread 1-6) WFLYMAIL0001: Bound mail session [java:jboss/mail/default]
2018-08-06 15:17:29,414 INFO [org.jboss.as.mail.extension] (MSC service thread 1-2) WFLYMAIL0001: Bound mail session [java:jboss/mail/whatever]