It makes more sense that you first check if the input is a valid e-mailaddress. If not, inform the user about this (422: invalid e-mailaddress). If the e-mailaddress is valid, you can check if this would lead to a conflict in the database (409: e-mailaddress is already registered).

Offtopic: I wouldn't return a 409 during the registration process though. For discretion purposes ;)