[Exim] filter: saving to /var/spool/mail/user

I'm trying out exim's filter rules for the first time, but haven't been able
to find an answer to my question in the manual or through experimentation.
I would like some of my mail to be saved to my regular "inbox",
/var/spool/mail/user. I tried using the "save" action, but that doesn't
work because exim needs to create a lockfile in the same directory, but
can't write to /var/spool/mail (which is 775, user root, group mail, so I
guess exim is trying to write as me).

I tried getting around this by creating a symbolic link from ~/Mail/inbox to
/var/spool/mail/user and making the rule "save Mail/inbox", but that didn't
work either, as exim apparently refuses to write to anything other than a
real file.

Any ideas how I can do this? TIA.


Nate