[exim] errors-to

Is there a reason why the "errors-to" parameter to the "deliver"
filter command is restricted to the system filter, even for trusted
users?

It would be useful to be able to use it in router-specific filters,
because some of my users want their mail delivered to gmail, and gmail
(a) honours SPF etc, (b) has sensitive spam triggers, so if I deliver
mail to gmail with the original sender address, it may well get
bounced. At present I have to have their personal procmails resend it,
but I'd much rather do everything in the router filter. (I have only
three users, and they don't want to touch our mail system:-)

I'm not sure what the security advantage is, as on most systems it's
trivial to manually reinject the mail with an arbitrary envelope
sender.

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