Is using a mail checker legal? Yes, absolutely. Checking your own email addresses is legal worldwide. However, using the Hackus Mail Checker Link to scan someone else’s email without permission may violate privacy laws (GDPR, CCPA, etc.) and Hackus’s terms of service.
Many modern tools (like the one you might be referencing) focus on "Magic Links" (passwordless login emailed to you). hackus mail checker link
Because Hackus relies on testing existing credentials, you can defend against it by following standard security hygiene: Is using a mail checker legal