there are few too many " in your command.
try using tab completion in CLI after /subsystem=mail/mail-session=
i think it should be something along the lines:
Thanks but this unfortunately also does not work. Same error as before
Should you mail session not have assigned a name so that it is found by jboss-cli?
What does your jboss-cli show when you enter
and then enter the tab key?
It completes to /subsystem=mail/mail-session=java\:jboss\/mail\/Default