Hi itzmie,
you're welcome!
The E-Mail Subscription is something every user has to decide for himself. So yes you can set this up in your user account on the place where you posted the screenshot from. Its not possible to do this as standard for every user without a Plugin. You could set this manually for all current users via the Database directly but at least here in Germany it's not allowed to put users without their permissions on newsletter-like lists like topic or forum subscriptions, so you could get some problems if you just add the users without giving them the chance to opt-in for themselves.
I recommend that you put up a global announcement where you give the users a info where they can set up the e-mail subscription if they want to, so you will not get any problems and you don't have to develop a Plugin to do it for you (or change it in your database every time someone new registeres).
The Mail Subscription feature however is set up so, that a user only gets ONE mail by default until the user reads the topic again.
So the CF3 handles Topic and Forum abos so, that a user only gets ONE mail even if multiple users reply. If a user subscribed to a forum and a topic in that forum the maximum is 2 mails until he reads it (one for new topic / answer in a forum, one for the reply on a specific topic those both subscription methods are not merged).
This will only be reset if the user logs into the forum and reads the topic. If a reply will be posted after this he will get another mail.
So if you get multiple mails for one single topic subscription even if you don't read the topic in between there is maybe a technical problem with your forum itself or with a Plugin you use because the standard way of sending mails is just one-until-read - also to prevent spam and overload of the e-mail system. Please also keep in mind, that a single topic-abo always gives just one notification about a new reply while a forum-subscription will work basically like those "Yahoo Groups" in the early days of the web. Some Users want Mails for everything so the subscription model gives the chance to decide wich type of subscription someone wants. So if you want to get less notifications a single topic-abo would be a better choice than a complete forum abo.
Sincerely,
Chris
