parent was talking about a different problem. to use your framing, how you ensure that in the email sent to the proper to/cc/bcc as you said there is no confidential information from another email that shouldnt be sent/forwarded to these to/cc/bcc

The restricted list means that it is much harder for someone to social engineer their way in on the receiving end of an exfiltration attack. I'm still rather skeptical of agents, but a pattern where the agent is allowed mostly readonly access, its output is mainly user directed, and the rest of the output is user approved, you cut down the possible approaches for an attack to work.

If you want more technical solutions, put a dumber clasifier on the output channel, freeze the operation if it looks suspicious instead of failing it and provoking the agent to try something new.

None of this is a silver bullet for a generic solution and that's why I don't have such an agent, but if one is ready to take on the tradeoffs, it is a viable solution.