Quoting Andrew Brown (atatat@atatdot.net):
> >Sorry for the wide distribution,
> >
> >Is there any way in sendmail to disallow incoming mail from a place
> >whose IP address does not map to a FQDN?
> 
> off the top of my head, something like this might do what you want...
> 
> LOCAL_RULESETS
> SLocal_check_rcpt
...
> R<$*> <$*>		$#error $@ 5.7.1 $: "550 no dot..."

For the record, there is zero requirement that reverse DNS
be there.  So if you perm fail (5.7.1) because the DNS
didn't look up, you're doubly wrong and breaking SMTP.

1) temp fail it, DNS servers do get unreachable or crash.
   Heck, if YOUR dns server eats it, you perm bounce all mail.  Sweet.
2) Don't do it. It's bad.  It's a bad idea.

I have some rules that add a HEADER (in 8.12) if there is
not reverse DNS.  I then filter with procmail on header:

into a folder.

I get a bunch of spam into there.  I also get a LOT of mail.
Some from some quite notable and aware Internet folks.
Folks who don't control their ISP's DNS.