So after much tweaking (and probably rejecting some email here and there.. )
Here's what I have that seems to be working.
drop message = return message text here
condition = ${lookup {$sender_address_domain}
cdb{/usr/local/etc/exim/broken_dsn_domains.cdb}{no}{yes}}
!verify = sender/callout
log_message = sender-failed [$sender_address - $local_part@$domain]
in the cdb is:
domain1.com
domain2.com
..