About on.bo

on.bo is a free, comprehensive email deliverability tester. Send a test email, get a full report on SPF, DKIM, DMARC, TLS, blacklists, subdomain visibility, and content. No signup. No paywall.

We built it because every other tool either stops at SpamAssassin, hides the interesting checks behind a subscription, or makes you stitch together five different sites to get the full picture.

The mission is simple: give every sender — solo developers to big marketing teams — the same diagnostics the experts use. Free, forever.