Ew no, don't always show success, there surely has to be a more elegant solution. I have multiple email addresses and sometimes I'll use a different one for a different service, if I enter email@website.com when I used email@anotherwebsite.com and you show success when it's the wrong one, I'll be waiting for an email that will never appear...
Maybe HN can work out a good solution collectively? I can only think of a secondary authentication piece, but then that's also possible to be forgotten.
How about show success, but adjust the confirmation e-mail accordingly. For example, if the e-mail address is not registered you could say something like "We received a request to recover your password on our service but see that you are not registered for our service with this e-mail address." ?
I considered this, but it makes an easy way for someone to cause the site problems. I guess coupled with captcha it could work, but someone could send a lot of emails to different people, if enough mark it as spam they might have problems as it is TECHNICALLY unsolicited, but I guess at the same time I could do the same with registering...
A possible solution is to get the email account holder to send an email to lostpassword@website.com. Auto-reply with a password reset form, or a welcome email in the case with no existing account (maybe you could fast-track account creation here, with a link "Oops, please delete this account. I didn't mean to do that.").
It doesn't really solve the problem of figuring out which email address you used to create your account, but it allows you to provide feedback to the user that the account does/does not exist.
An alternative solution would be to just email whatever account you enter on the website, with the appropriate wording depending whether or not they have an existing account, but this approach seems like it'd be too easy to spam random people.
Maybe HN can work out a good solution collectively? I can only think of a secondary authentication piece, but then that's also possible to be forgotten.