I once registered an account here at StackExchange, but only remember the username. How could I recover the email that belongs to that account, I tried several emails I have. None seem to work. Any chance?
You could email firstname.lastname@example.org with the user account you believe is yours. Telling you the email address is unacceptable, but they could likely send that account an email which you would receive (if the account is yours).
Another possibility is that if the account has a gravatar, you might remember which email address you signed up to gravatar with.