the following is how to monitor postfix on a linux or freebsd server.  It requires perl.
their is 2 files attached to this article.  One is the perl file.  the other is cacti graph template.
Items in bold have to to be typed form command line or ssh terminal.
You can put the fetch_mail_statistics.pl file anywhere you wish.  However wherever you put it you must make is executable.
chmod a+x fetch_mail_statistics.pl

Also you have to have net-snmp installed on whatever system you are trying to monitor.
After you have installed it and generated your snmpd.conf file then put the following in it.
pass .1.3.6.1.4.1.2021.255 /usr/local/bin/fetch_mail_statistics.pl /var/log/maillog /var/log/mailstats.db .1.3.6.1.4.1.2021.255

You have to change 2 things in here.  First change the path to your perl file to wherever you put it.  Next change the maillog location to wherever your maillog for postfix is defined as.

Then restart the snmp service.  Now you have to import the graph into cacti. Then go to data sources and add a new data source for your new device and add all the postfix ones.  Then add a graph and add to your graph all the data sources that you just created.