Summary: Ref T3472. Currently, if an install only allows "@mycompany.com" emails and you try to register with an "@personal.com" account, we let you pick an "@mycompany.com" address instead. This is secure: you still have to verify the email. However, it defies user expectation -- it's somewhat confusing that we let you register. Instead, provide a hard roadblock. (These accounts can still be linked, just not used for registration.) Test Plan: See screenshot. Reviewers: btrahan Reviewed By: btrahan CC: aran Maniphest Tasks: T3472 Differential Revision: https://secure.phabricator.com/D7571