Slack does this exceptionally well. If you forget which accounts you have, you can put in an email address and it will email you a list of your Slack accounts. If you forget your password, you can get a magic link that automatically signs in through a deep link into the app, no password needed.
It's such a cool idea. If you can reset your password using only your email, there's no security reason you can't just log in with it. It might even be better, since you can then add more annoying steps to the password reset strategy.
