How to reject mail on secondary MX?

[Ken D'Ambrosio]

Hey, all.  I've got a primary -- my actual destination/IMAP server -- and, for
various reasons, a secondary SMTP server.  My primary only rejects mail to
invalid recipients (spam is dealt with internally).  I'd like my secondary to
reject invalid recipients, also.  I can bring over /etc/aliases and /etc/passwd
-- we're talking perhaps 15 addresses, tops -- but how do I get the secondary
to check against them before relaying to the primary?

Thanks,

-Ken