Rule to allow internal zip files/block external

[Richard Alexander]

I am currently running MS 4.26.7, SA 2.63.1, on red hat 9.0.  Because of
the recent flood of *.zip attachment viruses we currently block all the
standard attachments as well as all zip attachments.  Is there a way to
allow to local users to send zip files within our local site, while still
blocking external zip attachments from entering our system?

Thanks