How to use CustomValidator on ModalPopupExtender

Hi,

I am using a ModalPopupExtender, in this modal the user saves an email, but I need to validate that the email does not exists to avoid duplicates.

I can't allow the page to postback because the modal will hide, so I need a sample to do it async.

I've seen lots of samples of CustomValidator but not on a ModalPopupExtender or not checking database.

is there any simple sample to achieve this?

Thanks in advance