Securing the mails delivery
To secure your mail server and the delivery of your e-mails, we recommend that you use the following three protocols:
SPF
The Sender Policy Framework (SPF) allows declare your mail server as authorized to originate mail for your domain(s).
To configure it, you have a single dns record to add:
Type: TXT
Host: mail.example.com
Value: 1.2.3.4