How to use @Valid with custom validation?

Question

i am using hibernate validation annotation framework to validate my domain classes and validating my domain object with @Valid annotation as follows:

@RequestMapping(method = RequestMethod.POST)
    public String post(@Valid @ModelAttribute("person") Person person,BindingResult errors) {...}

and i was wondering if i want to make custom validation like checking if email exists in database

what i am doing right now is to have a property in the domain:

@AssertFalse(message = "{email.missmatch}")
private boolean emailExist;

and i set it in the post method:

person.setEmailExist(personDao.isEmailExist(person.getEmail()));

above was working with custom hibernate validator class i had to validate domains and i was calling the validator after setting the property

but now with @Valid, the validator is called before setting the property

so, is there's a solution to use @Valid with this case ? or it won't work, if it will not work please suggest me how to validate my domain class in this case instead of using @Valid, what to use ?

thanks in advance.