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/ {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.


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, {rcpt_to}, {original_rcpt_to} provide access to the SMTP envelope recipient (before and after any rewrites), {subject} is replaced with the Subject header, if it is present.

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:


In this case, message will be placed in the Junk folder.


In this case, message will be placed in inbox and will have '$Label1' added.