On my incoming email, Sender is never tech-embed%netbsd.org@localhost, but always
the name and email of the person who sent it, in the case of your post,
abs%absd.org.@localhost Thus can't filter it. If it was as you said it would
have been easy.
Thunderbird should filter posts to this list fine by adding a rule
for matching mail with 'Sender: tech-embed-owner%NetBSD.org@localhost'.