A number of major email services make use of SPF as part of their spam scoring mechanisms.
Once the policy decision is made to offer a SPF response, it is imperative that the record be constructed using the correct syntax.
Traditionally, SPF records have been a implemented as TXT resource records. While a specific SPF resource record type has been introduced, it is classified as experimental.
All smtp servers that filter on SPF can query with a resource type of TXT. Some newer smtp servers will also query with the experimental resource type of SPF. Thus, the safest route to interoperability is to use TXT records for SPF for greatest compatibility.
An example of the data portion of a SPF record that covers most situations is:
v=spf1 mx -all
The example above can be interpreted as:
email can be sent for this domain by any server listed with an MX record for this domain, email from any other source should be treated as invalid
Note the absence of double quotation marks. Examples often show quotes for readability. However, the record itself should not contain quotes.
It is also possible to specify other servers as valid sources of email for a domain. The examples and testing tools may be found at the various SPF reference sites.