If you have a email service installed on your ClearOS instance and other apps use this service, you should write a TXT record for your domain on your DNS service informing the address of the server where email is running.
Let's say you own the domain example.com and your email service is running on a server with address 126.96.36.199, so you should create a TXT record with this content.
v=spf1 ip4:188.8.131.52 -all
When using a third party the configuration is slightly different. You will create a TXT record with SPF that includes a TXT from email service you are using.
For example, if you are using the email service from Gandi, you should include the Gandi record in mailcust.gandi.net on your TXT record. The code below will validate emails of your domain sent by Gandi server.
v=spf1 include:_mailcust.gandi.net ?all
You can include the third party record and add your server address on the same TXT record.
v=spf1 include:_mailcust.gandi.net ip4:184.108.40.206 ?all