JAMES-3775 RSpamD ADR (#1115)
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+# 55. RSPAMD
+
+Date: 2022-08-10
+
+## Status
+
+Accepted (lazy consensus).
+
+Implemented.
+
+## Context
+
+In order to detect spam, James provide 3 mechanisms:
+
+- SMTP Hook: decide to reject or not before spooling on the SMTP level
+- Mailet: custom the mailet pipeline for changing the mail processing when detect spam mail.
+- Mailbox listener: based on user action, eg: move in/out the message to Inbox/Spam mailbox, then decide to report spam or not to separate Spam filtering system.
+
+For basic, we can base on username, DSN filter, domains, IP... for configuring black or grey list.
+If we want to analyse deeper in each message content, we need a more complex system.
+Currently, James is integrated with [SpamAssassin](https://spamassassin.apache.org/) to tackle this problem.
+We found it hard to operate SpamAssassin, and we had a performance issue.
+
+For more selection, James's repository provides the RSpamD extension, which provides the same way as SpamAssassin but another system - [RSpamD](https://github.com/rspamd/rspamd)
+
+A quick introduction about RSpamD:
+
+```
+Rspamd is an advanced spam filtering system and email processing framework that allows evaluation of messages by a number of rules including regular expressions, statistical analysis and custom services 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 allowing direct interaction with popular MTA systems, such as Postfix or Sendmail.
+
+Rspamd is designed to process hundreds of messages per second simultaneously, and provides a number of useful features including a comprehensive Lua API that allows access to messages processing in various aspects as well as asynchronous network API to access external resources, such as DNS, HTTP or even generic TCP/UDP services.
+```
+
+## Decision
+
+Set up a new maven project dedicated to rspamd extension. This allows to be embedded in a James server as a soft dependency
+using the external-jar loading mechanism. With this way, the extension could be dropped in one's James installation, and not a runtime dependency.
+
+Based on James' support for custom mailets, listeners, web admin routes, RSpamD extension can be done via:
+
+- `RSpamDScanner` mailet: with each mail income, this mailet will query to RSpamD for getting a spam or ham result, then append new headers to the mail with status/flag spam.
+By setting up with `IsMarkedAsSpam` matcher, the mail will be rejected or not.
+This mailet will be configured in [mailetcontainer.xml](/server/apps/distributed-app/sample-configuration/mailetcontainer.xml).
+
+- Web admin route: to create feeding ham/spam messages task (batch mechanism). It helps spam classify learning.
+
+- `RSpamDListener`: the listener will handle mailbox events, based on `MessageMoveEvent`, `MailboxEvents.Added` to detect if the mail is spam or ham, then report to RSpamD,
+enrich data to RSpamD, thus we will get more exact results in the next query.
+This listener will be configured in [mailetcontainer.xml](/server/apps/distributed-app/sample-configuration/listeners.xml).
+
+To connect to RSpamD, we use http protocol with reactor http client.
+
+## Consequences
+
+- For higher performance, lower latency, the RSpamD should run in same network with James.
+- The query to RSpamD will get different score for same message.
+- A distributed mode for RSpamD is allowed by the use of Redis.
+
+## Alternatives
+
+- Rspamd can act as a [milter](https://en.wikipedia.org/wiki/Milter), we can use it to replace HTTP call. However, a milter client in James is harder to implement.
+
+## References
+
+- [JIRA](https://issues.apache.org/jira/browse/JAMES-3775)
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