Enforcing SPF Records
In order to enforce SPF records, changes to the MTA that receives messages for your users must be made. SPF checks should occur on the (mail relay) machines, which receive messages from the Internet for your users. Otherwise, the envelope information is lost, and SPF enforcement is moot.
In our SPF enforcement example, we use Sendmail. The SPF downloads page (http://spf.pobox.com/downloads.html) contains links to various SPF enforcement implementations for a number of MTAs.
spf-milter is an SPF implementation using milter for Sendmail written in Perl. It requires the following:
Perl 5.8.x or higher, thread enabled (compiled with -Duseithreads)
The following Perl modules: