Verifies the given sender address against the information in the last (most recent) received header line
Specifically, this does the following:
get domain of sender
get IP address of last (most recent) MTA
get hostname of last (most recent) MTA
if (MTA IP Address resolves to MTA hostname) then
use MTA hostname for MX IP records
else
use MTA IP Address
get MX records for sender domain
if(at least 1 MX record IP matches MTA IP) then valid