Qmail allows a maximum of 10 local and 20 remote email deliveries/processes simultaneously by default. The concurrencylocal and concurrencyremote files are the ones which handles the concurrent local and remote email deliveries respectively.

To increase the number of email deliveres/processes, you have to increase the values in the concurrencylocal and concurrencyremote files and restart the Qmail service.

The concurrencylocal and concurrencyremote files resides in the /var/qmail/control directory. If the files are not present, you have to create them

PHP Code:
# vi /var/qmail/control/concurrencylocal

# vi /var/qmail/control/concurrencyremote 
set the required value and restart the qmail service

PHP Code:
# service qmail restart