SPF is easy to add to your domain, it's just a TXT record to add to your DNS. When some spammer decides to send spam using your domain in the from address, ISPs who check SPF records will reject it on the spot so you don't get the bounces.
SPF stops forgeries.
The next step is to setup your MX to test if incoming mail is forged.
this along with greylisting really cuts incoming spam.
The problem with SPF is it breaks forwarding. Email Forwarders need to switch to "remailing".
Code:
steve@maggie:~$ host -t TXT wiseburn.net
wiseburn.net TXT "v=spf1 a mx -all"
If you start getting bounces from people who have forwarding setup, change that "-" to a "?".
the more domains that setup SPF records, the better it will work.
Yahoo! has a competing methodology called domain keys. Microsoft is working on "sender ID", however they still publish SPF records.
Steve