It strikes me that all other answers to this old question are link-only. So I am going to describe in few words how to implement my favorite solution for this job.
The link that @84104 has suggested, although very useful, cannot be used to properly implement rate per sender.
smtpd_client limits are not for traffic control but rather to help with client software: "Measures against clients that make too many connections". Although one can find recommendations such as this one that it surely may help. As @E. Yazici has suggested, an addon for postfix is required. Personally I find policyd as recommended by @Janne Pikkarainen rather cumbersome although it is considered a standard.
My favorite addon is postfwd because it is light and easy. It is also worth noting that this works very easily with Plesk or other similar, without affecting Plesk's complicated set of configuration files. Email rate limit in Plesk has been only implemented in version 12 but still the policy features are limited.
First download the latest addon from the site above. I believe there is no rpm for CentOS, in contrast to Ubuntu and Debian. Nevertheless, it is advised that the latest version is used. More specifically, version 1.32 (which is present in eg. Ubuntu 14.04LTS repos) has a nasty bug that prevents it from functioning appropriately. Version 1.35 sorts this out.
Make sure that these PERL modules are present. If using Debian or Ubuntu you can install from repos so that all dependencies get automatically sorted out and then replace
/usr/sbin/postfwd with the latest version.
Then create the ruleset. Create a file such as
/etc/postfix/postfwd.cf and add:
id=R001; sender=~/.*/; action=rate(sender/100/86400/REJECT only 100 messages per day for $$sender)
id=R002; sender=~/.*/; action=rate(sender/50/3600/REJECT only 50 messages per hour for $$sender)
The above ruleset has obviously two rules that evaluate for all senders. The syntax of the
rate action is:
rate (<item>/<max>/<time in sec>/<action>)
Other rate examples can be found here. Reference to the syntax can be found on the documentation. Similar discussion can be found here. If SASL is implemented (eg dovecot) you can safely replace
sasl_username. You can test the validity of the ruleset with the
postfwd -f /etc/postfwd.cf -C
Afterwards, you can optionally create a dedicated user and group
postfwd under which postfwd will run and launch it:
postfwd --daemon -f /etc/postfwd.cf -u postfwd -g postfwd
In case of having it installed from apt (Debian, Ubuntu etc), there should also be a configuration file under
/etc/default/postfwd and you could start the service properly, eg
sudo service postfwd start.
Then take a look at the log to verify that postfwd is listening. Postfwd uses the same log as postfix (eg
/usr/local/psa/var/log/maillog etc) and a line as
postfwd 1.35 ready for input should be in place.
Then, let postfix know to use postfwd. Edit the postfix conf file (usually
/etc/postfix/main.cf) and in the line:
smtpd_recipient_restrictions = permit_mynetworks,...
check_policy_service inet:127.0.0.1:10040. Please consider that the order where you put this within
smtpd_recipient_restrictions has great importance and you could end up spending a great deal of time diagnosing what could be wrong. As explained in this question, if one check returns OK or REJECT then postfix does not continue to the next one, so you should probably place this high.
Last, to check that this works you can either specify a very small limit such as 1 or even add a rule as
id=DEFAULT; action=dunno. Any rule hit gets logged anyway. Please also note that the rate limit per sender does not distinguish between mulitple emails with a single recipient or a single email with multiple recipients.
Then send an email from an account (in that server) and look at the log:
grep "RULES" /var/log/mail
Other links: postfwd quickstart.