Change admin email according to field input

You would need to have the values that a user supplied during registration in order to be able to have that logic in your filter. However, the only thing passed to that particular filter is the admin email.

The wpmem_post_register_data action hook comes right before the email process and has access to the submitted data as an array. I would created a function hooked to that action and in that function globalize the $fields so you can pick them up in your filter as a global.