Five tools that give you a pass/fail answer with a reason, not just a checkmark — validating an email address, checking a domain's mail configuration, or testing whether a password is actually strong.
Email validation
Email Validator checks an address against the standard email format and reports why it failed when it does — a missing domain extension, an invalid character, a missing @ symbol. Email Malformed Checker runs the identical validation logic under an earlier name kept alive because removing it would break an existing link.
Password strength
Password Strength Checker scores a password locally as you type, without ever transmitting it anywhere — the scoring happens entirely inside your browser.
The two tools that leave your browser
SPF Checker looks up a domain's public SPF DNS record to confirm it's configured correctly — that's a live lookup against DNS, not something a browser can do without contacting a name server. Dead Link Checker fetches a URL through our server to test whether it responds, since browsers block that kind of cross-origin request from client-side JavaScript. Both are genuinely different from the rest of this category, and each says so on its own page.
What "checking" doesn't mean
None of these tools guarantee an outcome — a valid SPF record doesn't guarantee inbox delivery, and a strong password score doesn't guarantee an account can't be compromised some other way. They catch the specific, well-defined thing each one is built to catch.