[BlueOnyx:13974] ban or jail <may be forged> messages

[wcstaff at webcoast.com]

I know this is an arguable scenario, but I am sick of the spam getting
through that is <may be forged>. 
It seems like spammers are figuring ways to forge and get around blocks.
I have recently found IP addresses that I force blocked, skating through on
a <may be forged>.
I read where some settings can be added to fail2ban that will reject these
emails. I tried adding them and it did not help.
I was also reading that Sendmail 8.4 has the feature
"FEATURE(`require_rdns')dnl", than can be enable or disabled. 
My question is how do I turn on and off this feature? I looked in
sendmail.mc and "FEATURE(`require_rdns')dnl" does not exist.
Thanks
Tom