A daemon which provides TLS client policy for Postfix via socketmap,
according to domain MTA-STS policy. The current support for RFC8461
is limited - the daemon lacks some minor features:

* Fetch error reporting
* Fetch ratelimit (but actual fetch rate partially restricted with
  `cache_grace` config option).

The server has a configurable cache backend which allows storing
cached STS policies in memory (`internal`), a file (`sqlite`) or
in a Redis database (`redis`).