Five agents hand over to one another on the same Gmail inbox. The handover stops at the review agent: nothing goes out without a round trip through the interface, and a rejection sends the message back to drafting.
Internal stepExternal serviceCheckpoint — review or opt-inVerified outputApplication boundary
Gmail is both the source and the destination: the loop closes when agent 5 puts the reply back. The two amber blocks mark the only place where a human steps in — without their verdict, the chain stops or goes back to drafting.