[Exim] SMTP/POP3 Splits

Can anyone please help with this - its driving me mad!

I am running exim 3.36 serving email for virtual domains. This is being
handled as per the alias style example configuration and works well for
delivering mail to local popboxes. I have a need however to deliver SOME
emails for a domain to local popboxes and forward the remainder onto
another SMTP server (in this case an in house server on the customers
site) This is possible with sendmail by having a user@???
%1@doman-name-referring-to-alt-server. entry in a virtual users table.
There surely must be some way I can achieve this with exim?! I can have
the emails delivered to a local pop account containing a .forward filter
to alter the addresses but this will not handle Bcc copies as Bcc is
stripped from the header before this point. My last ditch attempt method
of handling this would be to rewrite the addresses at smtp time but I
really don't want to have to resort to that. Can I handle it in some way
with another director to send the mail on when it fails to find an entry
in the domains alias file? Or would it be possible to rewrite the
address with a system filter? - This would seem a kludge to me. I cant
believe that it this isn't possible with exim, just that I cant figure
out how to do it!

Any help greatly appreciated!

Doug.