I think I understand - You’re asking if there are ways of changing the notification control through email only by a special reply (similar to “+1” response).
There is currently no “special” reply by email to subscribe without liking or responding; For that action it’s only available through the web app, but it is a nice suggestion.
EDIT: TIL, this is possible (below). Even while following Discourse’s development pretty closely I still miss some of these real cool ninja features!