- It doesn't have to: just add the right hosts or even entire IP ranges to the string.
That doesn't ensure that mail forwarding is not broken - The issue with mail forwarding is based upon the source address of the machine sending the mail through to a SPF-aware mail server. If mail is forwarded from the original recipient address to a secondary SPF-aware mail server, the mail will be marked as illegitimate because the source address of the mail message, as seen by the receiving mail server, is that of the primary mail server. In order to prevent this, all mail servers must incorporate the Sender Rewriting Scheme (http://spf.pobox.com/srs.html) for forwarding mail when forwarding mail to SPF-aware mail servers.
perl -le "print unpack'N', pack'B32', '00000000000000000000001011010110'"