Courier Mail Server


ratefilter — Rate-limiting mail filter


filterctl {[start] | [stop]} ratefilter


Enable this filter to enforce a maximum number of messages accepted from the same sender.

ratefilter uses the following configuration files. Changes to the following files do not take effect until the filter has been stopped and restarted.


This file contains a single numerical value that sets the rate limiting time interval setting, in seconds. The default value is sixty seconds. Sixty seconds is also the minimum acceptable interval.


This file contains a single numerical value that sets how many receipients the same sender can send mail to, within the controlling time interval. The default value is 100 recipients.


When ratefilter is enabled for locally originating mail, this sets the minimum system userid that has rate limiting enforced. The default value is 100. Mail sent by processes with lower uids are not subject to rate limiting.


If this file exists and contains the word "all", ratefilter creates its listening socket in /var/spool/courier/allfilters, otherwise the socket gets created in /var/spool/courier/filters, see courierfilter(8) for more information.

The enablefiltering settings controls whether local or ESMTP mail is subject to rate limiting, see courier(8) for more information.

In this context, local mail refers to mail that gets sent when a process executes the sendmail(1) command. This does not include mail sent via connection to local port 25, this is considered ESMTP mail.

Local mail gets rate limited based on the sending process's userid. Each numerical userid gets treated as an individual sender. ESMTP mail's sender is taken from the authenticated ESMTP's userid. Non-authenticated ESMTP mail is not subject to rate limiting.

Rate limiting is implemented by dividing chronological time into intervals that are half the time interval given in the ratefilter-interval setting. For example, with the default interval of sixty seconds, chronological time gets divided into thirty second intervals.

ratefilter counts the number of receipients in each message (and not just the number of messages) from each sender, in each time interval.

ratefilter rejects the message when the total number of recipients from all messages from the same sender in the current and the previous time interval exceeds the ratefilter-maxrcpts setting. The rejected message's number of recipients also get counted, for the purpose of rejecting future messages.