/usr/share/jetty8/lib/jndi/javax.mail.jar is in libjetty8-extra-java 8.1.3-9.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/jetty8/lib/jndi/../../../java/gnumail.jar
/usr/share/jetty8/lib/jndi/../../../java/gnumail.jar