One cannot access the email addresses of other users on Bitbucket. Email addresses are considered somewhat private.
That (deprecated) endpoint is used to retrieve you own address (in which case the username can be omitted from the URL), or the email address registered on a team account that you are an admin of.
Note that the latter is no longer applicable, as we dropped the ability for team accounts to have email addresses.
To contact a team's admins, you can send a message to the team, which Bitbucket will relay to all admins. Messages can be sent through the INBOX page.
Note that the non-deprecated way to access the current user's email addresses is: https://api.bitbucket.org/2.0/user/emails
https://api.bitbucket.org/2.0/users/{user}/emails
active already? I ask 'coz I'm getting 404 and also API browser has no hits for keywordemail
under 2.0 Service – Explicable