Inbox Mailers claims to 3x your open-rate by sending newsletters literally as the user is in their email inbox, so the mail appears as they are reading.

"I wonder how they do that?", I ask, assuming it's based on some sort of AI-based "this is the time when this specific user typically reads their email, so we'll send it then".

But I'm not sure that's it.

The service is only open for US-based traffic only, so it makes me wonder what's different about US traffic compared to any others?