The Sender Policy Framework SPF (original website www.openspf.org is gone, read more about this issue here) is today used by many mail servers or SPAM filters to determine if an email sent as originating from
firstname.lastname@example.org really originates from
example.net or if it was highjacked by a spammer. An SPF-enabled receiving mail server checks both, the fully qualified domain name (FQDN) as specified in the HELO or EHLO announce message sent by the mail client, and the domain of the envelop email address (MAIL FROM) against the SPF record(s) stored in DNS for that domain.
As an example, I own two domains,
michaelonroad.de. I run a SMTP server on the domain
mail.ntecs.de. As such, I need two SPF DNS records for domain
# DNS zone for domain ntecs.de @ IN TXT "v=spf1 mx -all" mail IN TXT "v=spf1 a -all"
The first line tells the SPF enabled mail server that all email in the form of
email@example.com MUST exclusively be sent by one of the servers listed as a Mail Exchange (MX) record of the same domain. The MX record resolves to an IP address which is then checked against the IP address of the connection. The second line is used to check against the EHLO (or HELO) announcement. For example my SMTP server announces itself with a
EHLO mail.ntecs.de. Only the IP address of the
mail.ntecs.de server is accepted here in this case (or more specifically the IP address it's
A record resolves to).
For my second domain
michaelonroad.de I would only need one SPF record:
# DNS zone for domain michaelonroad.de @ IN TXT "v=spf1 mx -all"
This is because I use
mail.ntecs.de even for sending mail from
firstname.lastname@example.org. Also note that in my case I only run one server for both incoming and outgoing mail (for all domains).