This page describes setting a Sender Policy Framework (SPF) record in the DNS Zone file for the email on your domain.
If you use Gandi Mail, or send emails from your instance, this is the basic TXT record you may use, for example:
@ 10800 IN TXT "v=spf1 include:_mailcust.gandi.net ?all"
@ 10800 IN TXT "v=spf1 ip4:184.108.40.206/21 ip6:2001:4b98:c::/48 ip4:220.127.116.11 ip4:18.104.22.168 ip4:22.214.171.124 ptr ?all"
This Simple Hosting records is only used if you plan to send e-mails through your website (via contact form, etc..)
After you've submitted each of these changes, be sure to activate the new version of your zone file, and keep in mind that it will take about 3 hours for it to take full effect due to the standard DNS propagation delay.