How to add validation to a custom module?

[namhost]

I made my own module with a custom field called "To Address" as shown here:

      $return['my_field'] =  array
      (
        'FriendlyName'      => 'My Email Field',
        'Type'              => 'text',
        'Description'       => 'bla bla',
      );

How can I add my own custom validation so that I can make sure:

1. The email is valid and if it is,
2. Execute an external service using the provided email address and show a notification if it is not valid

Does anyone know how to add custom validation like this?