[exim] Limiting $header_to: to local users ???

I have an entry in my system_filter.exim file that sends a mail message
to the local user if a condition is true. Thus, for an outgoing
message, it is sent to $header_from: and for an incoming one, it is sent
to $header_to: This works fine when it is only addressed to one person,
but 'spams' when there are multiple people in $header_to: (no problem if
they are all local, but big problem if not).

How can I remove non-local addresses from $header_to: before using it?

Cheers
Kevin