There are two reasons for using spf: blocking faked sender id's and registering your own sender id for others to use in their setups.
It seems that spamassassin can handle spf lookups, I don't know if the sme server version can be configured that way.
spf.pobox.com offers a wizard to help you create the rules to implement spf. The result of running the wizard is a rule for inclusion in the tinydns config.
Where do I go from there? Do I have to edit any tinydns config (and if so... how)? Is spamassassin the way to go?
Has anyone gone beyond configuring dns on sme server by registering at spf?