I may have some insight here: in marketing there are third party services which will "verify" an email is legit. They do this by sending an automated inquiry to that email domain's servers. Many servers are set up to respond and verify, "yes this is a real address" or "no, we don't have an address like that."
Catch is that this allows marketers to confirm, yes, my message will reach a real person if I send it here, and they may send spam. So some services intentionally configure their servers to give no response or a less definitive response.
Without looking into it I would bet that's what Tutamail has done.
Still annoying that Loudly doesn't support it, but probably less a question of size/location and more of configuration.