[Exim] Warnings/Errors

[ Apologies if another very similar message shows up - I sent it yesterday,
but it seems to have gone down /dev/null... ]

I have a single-user machine on a private IP, IP-masqueraded to the outside
world, running exim. So whenever I send mail, my email address on that
machine gets rewritten to an externally-valid address. All fine, no problem.

However, this means that if (for example) a message ends up sitting in the
queue for a while, the warning messages get sent to that external email
address. If the delayed delivery is caused by some network failure, say,
the warning messages will probably get stuck in the queue as well, which is
a bit useless.

I've read FAQ Q1003, which is related, but doesn't quite solve the problem.
Ideally I'd like warnings to get sent to me on the local machine as long
as the message stays in the queue on the local machine...but when it does
eventually leave the queue to have the correct externally-valid envelope
sender. I suppose I'm asking for sender addresses (NB not necessarily
recipient addresses) to be rewritten at outgoing SMTP time, rather than at
receipt...and I realise exim doesn't do this!

However, a very acceptable alternative would be for *all* errors/warnings
just to go to a single address on this machine (it's single-user anyway).
[ The "errors_to" director/router option is the wrong thing for this,
because I don't want it to set the envelope sender - I just want errors
generated by the local machine to go to a specific address. ]

Any thoughts?

Thanks,
Chris.