notificationMailReceiverMail seems to be not working

[medarob]

I added the following email in the constants.typoscript

plugin.tx_formdoubleoptin_doubleoptin {
  settings {
    notificationMailReceiverMail = my@email.tld
  }
}

but there was no email sent to this address, once the double opt-in registration process has finished.
Furthermore I don't think that this setting as any effect at all. Is that possible? (I tested it with various emails to see if the email will get a notification).

In the constant settings this field is marked as mendatory.

The only way I could get an email sent after the double opt-in process has finished was via the settings in the form.

If I leave the constant unused/empty, no email will be sent but as the field is mendatory I would have expected an error or a warning in the form settings?