Sounds like a case for sometimes
$whitelist = ['admin@test.de', 'foo@bar.com', 'etc@etc.com'];
$validator->sometimes('email', 'unique:users', function($input) use ($whitelist){
return ! in_array($input->email, $whitelist);
});
Meaning if the email address is not listed in the white list, the rule applies unique.
Inside the form request, you can add material to the validation instance by overriding getValidatorInstance():
protected getValidatorInstance(){
$validator = parent::getValidatorInstance();
$validator->sometimes(...);
return $validator;
}