The address for 'subsystem=mail/mail-session=java:jboss/mail/Default' must be escaped because of the ':' and '/' characters in the 'mail-session' node.

Log in via the DMR cli shell and you're able to browse the same node via '/subsystem=mail/mail-session=java\:jboss\/mail\/Default/:read-resource' If you attempt to read the same node via escaped json you get:

failureDescription=JBAS014807: Management resource '[
("subsystem" => "mail"),
("mail-session" => "java\\:jboss\\/mail
/Default")
]' not found, rolledBack=true}