Hi Jon -
Yeah - I think the plugin does successfully read the user's email address. I know this because if I click to 'Insert Signature' in the editing window of a wiki entry, I get this:
--- //[[joeuser@gmail.com|joeuser]] 2016/01/19 10:30//
That user is a forum user, so it must be getting the email from the forum's SQL table ok. Just not sure where it stores it so that it would work for subscriptions. With Dokuwiki's internal authentication system, this data would be in a text file on the server somewhere I guess, so perhaps Dokuwiki looks in that file and finds nothing because the authentication is done in a different way?