Find every subdomain
of a domain.

Every hostname under a domain that ever appeared in a public TLS certificate — with certificate counts and first/last seen dates.

2 hosts · 156 ms

HostCertsFirst seenLast seen
track.sl.mail.twu556.org 2 2026-07-12 2026-07-12
mta-sts.mail.twu556.org 5 2026-05-07 2026-09-04

Programmatic access: API GET /v1/hosts/mail.twu556.org (API key for paging).

Shown here: hostnames with a certificate that is valid now or expired within the last 90 days. The full history since 2013 — hostnames whose last certificate expired earlier — is a Pro plan method: GET /v1/hosts-history/mail.twu556.org.