The Message-ID of outgoing e-mail sent via the default javax.mail.Session has the format

Message-ID: <524672585.11.1429091886393.JavaMail.wildfly@myserver.example.com>

The wildfly part here is not hard-coded, it corresponds to the user ID of the process WildFly is running under (which happens to be wildfly on my server).

Revealing the user ID of a system process may be regarded as a security risk.

This has been fixed in javax.mail 1.5.3 (see https://kenai.com/bugzilla/show_bug.cgi?id=6496), so WildFly should upgrade this dependency.