SPF is a rule in the DNS that lets the world know which servers are allowed to mail with your domain name in the email address.
When using Kerio, your MX record will usually point to the Kerio server. The following SPF record would then be sufficient: "v=spf1 mx a -all".
So add a TXT record to your DNS with the value: v=spf1 mx a -all
You can generate your own spf record on https://www.spfwizard.net/ook.