[Exim] Errors/Warnings

I have a machine which is IP-masqueraded through a firewall to the outside
world - so my email address on that machine is rewritten to an externally-
correct one whenever I send mail. No problem.

However, this means that if (for example) mail is delayed on the queue, the
warning message gets sent to my _external_ address (as you'd expect). If
the reason for the delay was that there's no external link at present, this
means that the warning will get delayed too. You get the picture.

I've read FAQ Q1003, which is related, but still doesn't quite solve the
problem. Ideally I'd like errors/warnings to come back to me on the local
machine while the mail is still in exim's queue, but still for the mail to
get sent with an externally-visible envelope sender address when it does
finally leave the queue. A very acceptable second-best would be for *all*
errors/warnings just to go to a single address on the local machine (e.g.
postmaster), but still for the mail to get sent with the externally-correct
envelope sender.

Is this possible?

Thanks,
Chris.