Validating incoming email addresses

[karlp at ourldsfamily.com]

How do I go about blocking incoming email based on validating the sender's
email address? I am getting spam email which is from a non-existent email
address on my own domain. A look at the header shows it's not from my
domain. I expected that. But, the From: field is from my domain
(ourldsfamily.com), even down to my server name (moroni.ourldsfamily.com)
which NEVER sends email, per se, other than internally as in mail
generated by cron jobs.

Too much information, but I hope you get the gist of what I need.

TIA,

Karl