SPF (Sender Policy Framework) is an anti spam method which is based on the authorization of the sender of an e-mail. The SPF record is additional information in the DNS settings of your domain. It defines which mail servers are authorized to send e-mails. This means:
You own the domain example.co.uk and you want to make sure that e-mails sent to your domain may only be sent via the 1&1 mail server. Set the SPF record:
"v=spf1 include:_spf.perfora.net include:_spf.kundenserver.de -all"
As a result, for example, the recipient of the e-mail firstname.lastname@example.org can reject the receipt if it has not been sent via a 1&1 mail server.
With the command all you can define how such e-mails should be treated: