First thing I found - email@example.com doesn't exist - but firstname.lastname@example.org does actually exist so it's not so silly an email.
This got a bit long, so tl;dr
- Pinterest - liars
- Google, Facebook, Trello (nicely) - all tell you if the account exists
- Slack - agree with you and gurvinder372's answer
Personally I'm a fan of Slack's UX in general, you could also combine it with some of the language from Trello.
One other thing I like but don't see done is just to do a DNS lookup of the email domain as that that might pick up spelling mistakes, but doesn't rely on any regex and doesn't release any private information.
The long bit...
Lying to customers is a bad thing. Please don't be offended, but I genuinely find it quite interesting how offended you are by it, rather than just assuming it was a developer who didn't think of catching the case of someone trying invalid email addresses.
I have a guess that they might be just re-printing the email you typed in back at you in an attempt to show you if you mistyped something.
Anyway, it's interesting to see what companies do, because forgetting passwords and emails is really frustrating and so UX folks should be very careful.
Facebook make you search for your account, I guess they figure that so many people have Facebook accounts that there's not much to be made by someone knowing you have a facebook account:
I thought Google was doing some kind of basic domain checking as it rejected email@example.com but accepted firstname.lastname@example.org.
However further checking showed that email@example.com isn't recognised by google. So it actually looks like firstname.lastname@example.org definitely is a real email address and they now think that someone is trying to hack into their account.
Pinterest are clearly just a bunch of liars (perhaps this was the site you hit):
Trello, follow along Facebook and Google lines of just telling you whether the account exists or not. However they're more friendly about their message, which is always a good thing:
Slack first assumed that bork.slack.com does exist - it seems that they'll accept anything I tried bananananananana.slack.com and it let me reset my password for that too.
We finally get to someone doing what you and gurvinder372's answer suggest. The wording is quite nice too:
If we found an account associated with that email address, you will find an email from us in your inbox shortly.
Unsure which email you used to create your Slack account? Please contact email@example.com.