1) Login to CWP (http://your-ip.com:2030)
2) Naviagte to Email –> Postfix Manager and tick mark Install DKIM & SPF: and click on Rebuild Mail Server
3) Now goto Email –> DKIM and SPF Manage and Select Domain Name, tick enable spf and then click Add DKIM
As you see in above image v=spf1 is not enabled yet. Lets enable SPF manually.
4) Navigate to DNS Function –> List DNS Zones and there you will see your domain name and infront of it there is Edit File button. Click Edit File and Add below entry.
your-domain-name. 14400 IN TXT "v=spf1 +a +mx +your-ip-address ~all"