SPF, DKIM and DMARC are 3 types of DNS (Domain Name System) record that authenticate your domain name, they tell the servers that your emails and are coming from your domain, and are ruled out as spam.
SPF stands for Sender Policy Framework.
The Sender Policy Framework (SPF) can be used to determine whether an email is actually coming from the domain server that appears as the sender. This authentication method allows mail servers to verify the authenticity of sender addresses, that it is actually from the specified host server. This SPF check runs automatically in the background without you having to do anything.
Put simply, the SPF determines which mail servers are allowed to send mail for the domain. The mail servers are identified by their name or their IP address. For example, an email from john.doe@gmx.com may only be sent via one of the following IP addresses: 213.165.64.0, 74.208.5.64, 74.208.122.0, 212.227.126.128, 212.227.15.0, 212.227.17.0, 74.208.4.192, 82.165.159.0, 217.72.207.0. These IP addresses are therefore listed in the SPF record for the domain gmx.com. The incoming mail server can then check whether the IP address that it reads in the header of the mail is on this list or not.
The list of authorized mail servers is stored on the domain name server (DNS) of the sending domain (gmx.com in this example) and can be accessed by any incoming mail server.
