Silverstripe Extension Example

/*
 * config.yml
 */
UserDefinedForm_EmailRecipient:
  extensions:
    - 'UserDefinedForm_EmailRecipient_Extension'

/*
 * UserDefinedForm_EmailRecipient_Extension.php
 */
class UserDefinedForm_EmailRecipient_Extension extends DataExtension
{
    public function updateFilteredEmailRecipients($recipients, $data, $form)
    {
        die("Getting ready to update some email recipients!");
    }
}

/*
 * Snippet from UserDefinedForm.php
 */
    public function FilteredEmailRecipients($data = null, $form = null) {
        $recipients = new ArrayList($this->EmailRecipients()->toArray());

        // Filter by rules
        $recipients = $recipients->filterByCallback(function($recipient) use ($data, $form) {
            return $recipient->canSend($data, $form);
        });
        $this->extend('updateFilteredEmailRecipients', $recipients, $data, $form);

        return $recipients;
    }