PowerMTA is no doubt great MTA but it requires lots of configuration to make it work intelligently. If your pmta license have Out bound connections limit than you have to configure it smartly, to free up connections and increase emails per minute/hour.
Most of the time, these connections are used by invalid domains e.g hotmaail.com, yahooo.com, gmail.cm and on other hand your bounce-after for global domains is too long e.g
bounce-after 4d 12hr (by default)
How PowerMTA works?
PowerMTA tries to resolve Domain to IP (If it fails, it doesn’t make tcp connection, through DnsQueryFailed error), then it tries to make tcp connection and wait for SMTP greeting. (It keeps trying until smtp-greeting-timeout value (default 5m) reaches.After5 minutes, connection is killed and this process is repeated until bounce-after (in your global domain configuration) is reached.