About the Inbox Checker
The Inbox Checker looks up the mail servers for the address, connects to them the same way another mail server would and asks whether the mailbox exists — without sending any message. It also flags disposable, role-based and catch-all addresses.
Some providers accept every address (catch-all) or hide mailbox status. In those cases the result is shown as unknown instead of pretending the address is valid.
Deliverable or not?
We test everything that can be tested from the outside — but only the receiving server knows if a message will reach the inbox. Results are PASS, WARNING, FAIL or UNKNOWN; we never claim more than the checks prove.