Yeah, the only thing you need for simple SMTP services is: - with DNS, the simplest as possible SPF record. - Without DNS, (aka pure IPv[46] SMTP), your have

Yeah, the only thing you need for simple SMTP services is: - with DNS, the simplest as possible SPF record. - Without DNS, (aka pure IPv[46] SMTP), your have implicit SPF: instead of querying the DNS and parsing the SPF record, you parse the mail header to check "reply-to/from/etc" fields (the appropriate fields) for the sending SMTP IPv[46] address, that to perform spam scoring.