getting key of submitted entity list field
I'm not familiar with entityform so forgive me if none of this makes sense, but I imagine it works much like webform in that it sends the e-mail to the addresses you set up upon a successful form submit with no need for any custom code. So adding another validator or submit function to send the e-mail would result in two e-mails.

What you can do is use hook_mail_alter in a custom module and see what variables entityform passes to its drupal_mail call. With any luck, all the form values from $form_state will be included and you can do the check there and then change the $message['body'] array accordingly.

In the hook_mail_alter you can do a switch statement for the $message['key'] or $message['id'] to make sure you're not running your check for every mail your drupal site sends but only for the entityform in question.

I would also include a custom token of sorts in the template you, presumably, entered when settings up the entityform so you can easily change the e-mail body once you've run the check. I don't know if you could do it with Drupal's Token system, but even just entering 'my_module_custom_token' in the template and then using str_replace on 'my_module_custom_token' with the correct sentence in the mail_alter is better than nothing.

If none of this applies, then naturally disregard this answer entirely.