Mail servers communicate with each other via a protocol called SMTP. In practice receiving mail server simply asks the details of the email message to the sender party and process the message solely based on the information provided. This leads to a security problem in certain cases. A sender can introduce himself as whoever he desires since it depends on the info he shared. So how can the receiving party be sure that the sender is authentic? There are a bunch of solutions and the most popular one is the Sender Policy Framework aka SPF.
SPF is a simple DNS record that include a text that includes the list of authorized email servers for the domain name. An example of SPF record is shown below;
v=spf1 a mx ip4:126.96.36.199 include:sumasamail.com -all
Sender Policy Framework (SPF) records have three main sections. The first part (v=spf1) is to identify this record as a SPF record. The last part states the level of binding of these records. “-all” means that do not honor any connection out of the scope of this record. Between these two sections we define the authorized mailers.
Sumasa needs you to set up these records properly to be able to send emails on behalf of your domain name. By setting these rules, we ensure the deliverability of email messages and keep our servers secure from blacklists. Sumasa will guide you about the format of this entry based on your existing Sender Policy Framework (SPF) records.
You can also read your questions about Sender Policy Framework (SPF) on Wikipedia.