You can’t do this without the assistance of a webhook. There’s no built-in feature to centralize your domain list, let alone check new people to see if their domain is in the list.
With a webhook in the mix, it’s relatively simple, since the webhook-compatible service can maintain a list of unique domains, and when you call it, it’ll return true/false for whether the domain is net new. You can trigger that response in the Webhook is Called trigger.
To be thorough, you might also “offboard” people by running them through a second related webhook before deleting them. That way if someone is the last person in the db with a current domain, you can delete that domain from the list, so seeing a future person from that domain is considered interesting.