The password reset logic confirms whether en email exists in the user database. This is generally considered bad practice as it enables malicious users to “query” discourse user db. DigitalOcean is a good example of being vague about the provided email, “if the email you specified exists in our system, we’ve sent a password reset link to it”
If you want this, enable it in site settings. Already exists.
By default we tell people since they can never remember which of their 10 email addresses they signed up with, so it is user hostile to force them to walk through all 10 emails to figure out which one they used on your site.
The signup form already gives away if an email address already exists (and turning that off doesn’t make sense) so it doesn’t really matter here, it doesn’t make that attack vector go away and it only makes things harder for users who actually forgot their password (or: forgot their email address)
@codinghorror thanks I didn’t know there’s a setting for it
@michaeld yes, it is true that the sign up form confirms for the existence of an email but one can reduce the email visibility by integrating Discourse with external identity providers or creating a custom sign up form with CAPTCHA.
There are already rate limits on these forms, try it yourself.
I suppose this was an old post, but in case anyone else runs into this issue and is worried about people being able to fish for emails of existing community members, looks like discourse implemented a simple fix for that. In settings search for “hide email address taken” and enable it. This will prevent users from using the “forgot password” to query for emails in your server.
Just to link this up, it was added in this commit as part of this topic - Hide 'email account exists' for invites 
Nope, I wasn’t paying attention. 
That was to extend that setting further to include the invite screen. Still, an interesting addition that I’m glad I mentioned.
Closed in favour of Email enumeration vulnerability on "Password Reset" dialogue
