such as URL black lists. Each message is analysed by Rspamd and given a verdict that might be used by MTA for further processing (e.g. to reject a message, or add a special header indicating spam) along with other information, such as possible DKIM signature or modifications suggested for a message.
Rspamd can act as a [Milter](https://en.wikipedia.org/wiki/Milter) allowing direct interaction with popular MTA systems, such as Postfix or Sendmail.
useful features including a comprehensive [Lua API](https://rspamd.com/doc/lua/) that allows access to messages processing in various aspects as well as [asynchronous](https://rspamd.com/doc/lua/sync_async.html) network API to access external resources, such as DNS, HTTP or even generic TCP/UDP services.
Rspamd is [packaged](https://rspamd.com/downloads.html) for the major Linux distributions, and is also available via [FreeBSD ports](https://freshports.org/mail/rspamd), NetBSD [pkgsrc](https://pkgsrc.org) and [OpenBSD ports](http://openports.se/mail/rspamd).
We advice to use packages provided by Rspamd project if available for your OS instead of packages that might be provided by some Linux distributives, as they are usually out of date and does not provide the desired spam filtering quality nor supported by Rspamd project.
If that is not enough, Rspamd provides an extensive [Lua API](https://rspamd.com/doc/lua/) to write your own rules and plugins: <https://rspamd.com/doc/tutorials/writing_rules.html>