The Sender Policy Framework (SPF) is an open standard specifying a technical method to prevent sender address forgery. Our customer's domains have published a SPF record to specify which servers are the unique servers that are legitimate to send email for that domain.
Those servers are indicated by a TXT DNS record, so any mail server that receives an email from one domain of our customers, can check that record to verify if the origin server is authorized to send messages for that domain.
In our incoming mail servers, we check SPF, so if you publish your records in your domain's DNS settings, we'll check if mails that claims to be sent from your domain are forged or not. If you don't know how to do this, you can use the wizard located in the SPF's website.