Email validator
Bulk-check a list of addresses: one line each, clear per-address verdicts, no silent "looks okay to me". Syntax problems are named precisely; near-miss domains get a typo suggestion; and an optional live MX check confirms each domain actually publishes mail servers.
Syntax checks run entirely in your browser — nothing is uploaded.
Testing sign-up flows instead?
A validator tells you an address could work; a real inbox tells you the mail did. Tempx gives every test run a disposable inbox with a free JSON API — send to it, poll it, assert on what arrived.