Is PowerMTA ever getting the connection/message? To troubleshoot, enable inbound logging in PowerMTA via "log-commands yes" in the appropriate <source IP> definition, for example;
<source a.b.c.d>
always-allow-relaying yes
log-commands yes
</source>
replacing a.b.c.d with the IP of the machine running the bin command. Remember to save the config and reload PowerMTA after making the change and before your test. Then submit a message and see if anything goes into the PowerMTA log file. If not, you are not communicating with PowerMTA.
Since Postfix is getting the messages, the odds are very high that Postfix is still running and getting the messages directly from the app, not PowerMTA.