I'm a little late to this party, but…
Here's a couple of examples of well-known domains that "fail" my check:
warn
condition = ${if and{{def:sender_host_address}{!def:sender_host_name}}\
{yes}{no}}
add_header = X-Host-Lookup-Failed: Reverse DNS lookup failed for
$sender_host_address (${if eq{$host_lookup_failed}{1}{failed}{deferred}})
cambridgejazz.org
thegreatcourses.com
localsecrets.com
mbna.co.uk
and that includes real user account emails, not just marketing spam.
apols for the late reply.
cheers,
calum.
