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docs/reference/storage/imap-filters.md

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+# IMAP filters
+
+Most storage backends support application of custom code late in delivery
+process. As opposed to using SMTP pipeline modifiers or checks, it allows
+modifying IMAP-specific message attributes. In particular, it allows
+code to change target folder and add IMAP flags (keywords) to the message.
+
+There is no way to reject message using IMAP filters, this should be done
+eariler in SMTP pipeline logic. Quarantined messages are not processed
+by IMAP filters and are unconditionally delivered to Junk folder (or other
+folder with \Junk special-use attribute).
+
+To use an IMAP filter, specify it in the 'imap\_filter' directive for the
+used storage backend, like this:
+```
+storage.imapsql local_mailboxes {
+   ...
+   
+   imap_filter {
+       command /etc/maddy/sieve.sh {account_name}
+   }
+}
+```
+
+## System command filter (imap.filter.command)
+
+This filter is similar to check.command module
+and runs a system command to obtain necessary information.
+
+Usage:
+```
+command executable_name args... { }
+```
+
+Same as check.command, following placeholders are supported for command
+arguments: {source\_ip}, {source\_host}, {source\_rdns}, {msg\_id}, {auth\_user},
+{sender}. Note: placeholders in command name are not processed to avoid
+possible command injection attacks.
+
+Additionally, for imap.filter.command, {account\_name} placeholder is replaced
+with effective IMAP account name.
+
+Note that if you use provided systemd units on Linux, maddy executable is
+sandboxed - all commands will be executed with heavily restricted filesystem
+acccess and other privileges. Notably, /tmp is isolated and all directories
+except for /var/lib/maddy and /run/maddy are read-only. You will need to modify
+systemd unit if your command needs more privileges.
+
+Command output should consist of zero or more lines. First one, if non-empty, overrides
+destination folder. All other lines contain additional IMAP flags to add
+to the message. If command wants to add flags without changing folder - first
+line should be empty.
+
+It is valid for command to not write anything to stdout. In this case its
+execution will have no effect on delivery.
+
+Output example:
+```
+Junk
+```
+In this case, message will be placed in the Junk folder.
+
+```
+
+$Label1
+```
+In this case, message will be placed in inbox and will have
+'$Label1' added.
+