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TLS & SSL

SMTP TLS Checker

Probe a mail server's STARTTLS support, negotiated TLS version, cipher suite, and certificate chain validity.

Common ports: 25 (SMTP), 587 (Submission), 465 (SMTPS). We attempt EHLO + STARTTLS and report capabilities, AUTH methods, and an open-relay heuristic.

About this check

Probe STARTTLS, TLS version, ciphers, and cert validity.

Frequently asked questions

What does the SMTP TLS Checker actually check?
Probe STARTTLS, TLS version, ciphers, and cert validity.
Is this really free?
Yes. Every on.bo tool is free, no signup required, no rate-limit tricks.
Do you store the data I check?
We log minimal metadata to keep the service healthy. Lookups themselves are not retained beyond short-term caching.