check_qmailq

The original plugin is from http://www.openfusion.com.au/labs/nagios/
Download the check plugin:

The check command takes the syntax of check_qmailq -w count -c count [-W pre_count] [-C pre_count] [-q /path/to/qmail-qstat]. The warning and critical message counts can be adjusted to your preference. Edit /usr/local/nagios/etc/nrpe.cfg and add entries for the services you want to check, such as:

Restart NRPE
Nagios user permissions to access Qmail queue
In order to access the Qmail queue, the Nagios user needs to be added to the qmail group. Since that user only has read access to the queue, this isn't a security risk (or at least much of one...). Edit /etc/group:

Nagios Server Configuration
Since you should have figured out how to install a Nagios server in the first place, I'm not going to provide you with instructions on how to configure a host or service entry for your mail server other than the syntax for the service check_command would be check_nrpe!check_qmailq