When does Header chech take place?

[unruh]

I have machine A relaying mail to machine B. I am getting spammed on my
"addresses" from my newsnet postings, slrn*unruh@wormhole
where * is some 8 random digits. Of course machine B rejects these as
invalid addresses, and the mails are then piling up on machine A as it
tries to deliver them.
I tried to put an entry into header_checks of
/^To: slrn.*unruh@wormhole/ DISCARD
to discard such addresses, but they are still getting through. When does
postfix do the header check and does it get done before the mail is
relayed to the other machine? This is annoying because my mailq gets
clogged up.
I do have
header_checks = regexp:/etc/postfix/header-checks
in main.cf