Thanks for the detailed report.
Let me just clarify something first. RBE does not send any emails. bbpnns does when sending new topic/reply notifications. RBE hooks into bbpnns and sets the correct Reply-to address in bbpnns’ outgoing message. Then RBE connects to the inbox and looks for messages with the expected plus-alias.
I think it’s time I had a look around myself. Can you please set up a staging site and create an admin account for me? It’d be great if you could also get me SSH access so I can run my test scripts.
We can switch to email communication from here. Please email me at vinny [at] usestrict [dot] net.