Sender Policy Framework (SPF)

Sender Policy Framework (SPF), is an e-mail validation system designed to prevent e-mail spam by addressing a common vulnerability, source address spoofing. SPF allows administrators to specify which hosts are allowed to send e-mail from a given domain by creating a specific DNS SPF record in the public DNS..

example.org. IN SPF "v=spf1 a mx -all"

"v=" defines the version of SPF used. The following words provide mechanisms to use to determine if a domain is eligible to send mail. The "a" and "mx" specify the systems permitted to send messages for the given domain. The "-all" at the end specifies that, if the previous mechanisms did not match, the message should be rejected.
[edit] Mechanisms

Eight mechanisms are defined:
ALL     Matches always; used for a default result like -all for all IPs not matched by prior mechanisms.
A     If the domain name has an address record (A or AAAA) that can be resolved to the sender's address, it will match.
IP4     If the sender is in a given IPv4 address range, match.
IP6     If the sender is in a given IPv6 address range, match.
MX     If the domain name has an MX record resolving to the sender's address, it will match (i.e. the mail comes from one of the domain's mail servers).
PTR     If the domain name (PTR record) for the client's address is in the given domain and that domain name resolves to the client's address (forward-confirmed reverse DNS), match.

EXISTS     If the given domain name resolves to any address, match (no matter the address it resolves to). This is rarely used. Along with the SPF macro language it offers more complex matches like DNSBL-queries.

INCLUDE     If the included (a misnomer) policy passes the test this mechanism matches. This is typically used to include policies of more than one ISP.