I'm sorry if I'm being unclear, but that's not at all what I'm requesting.
Let me start all over and use an example from non-discourse world that is the behavior I've come to expect from any website with a signup/login.
I have two email addresses:
Let's say I signed up for an airmiles program with email@example.com, but I haven't flown that airline in 15 months, so when I come back to sign in, I assume I probably used firstname.lastname@example.org and enter that as the username with whatever I think my password is.
I expect to see "No account matches email@example.com" not "If an account matches firstname.lastname@example.org, you should receive an email with instructions on how to reset your password shortly." (what discourse does) Granted, I would see that, wait 15 minutes, get no email and try email@example.com instead, but we've found our user base doesn't necessarily follow that logic.
And following the logic I laid out:
If I enter firstname.lastname@example.org, it displays an error message "No such user found" and no email is sent
If I enter email@example.com who is not a user, it displays an error message "No such user found" and no email is sent
If I enter firstname.lastname@example.org, I get the reset message
If I enter email@example.com, he gets the "You (or someone else)" message and ignores it - and I think the limiting rules outlined above cover that scenario well to avoid obnoxious abuse and the user receiving the erroneous email is already signed up for that specific forum, so it's not an unsolicited alien service
If I enter firstname.lastname@example.org on zombo.com, that falls into "If I enter email@example.com who is not a user, it displays an error message "No such user found" and no email is sent"
Nobody gets an email unless they are a user, the thing I'm trying to shore up is clarifying for people who don't receive an email that it's because they entered the wrong thing, not because the site went down or it's lost in their junk folder or something.