OT: block mail for specific user

[James Pifer <jep obrien-pifer com>]

This is way OT, but I'd rather not get on yet another mailing list. I
want to setup email for one of my kids, but I only want to allow mail
from specific addresses and blocking all others. 

I'm running Sendmail with MailScanner. Any way to do what I'm looking
for?

Thanks,
James