SPF records are a mechanism that allows you to define, via the DNS service, which servers are authorized to send emails on behalf of your domain.
For example:
domaine.com. IN TXT "v=spf1 a mx include:fwdsc.swisscenter.com -all"
If we break down this record:
domaine.com is the domain name concerned
TXT is the record type (Text in this case)
v=spf1 is the SPF version used
a authorizes the IP address of the domain itself
mx authorizes the servers defined as MX (email receiving servers)
include:fwdsc.swisscenter.com includes the record corresponding to the list of SwissCenter's SMTP servers.
-all defines the measures to be taken in case of SPF failure (we recommend the - sign, which implies strict handling: anything not sent by an authorized server must be rejected)
You can also use an online generator, for example: https://mxtoolbox.com/SPFRecordGenerator.aspx
The console already publishes an SPF record of this kind when the DNS service is enabled for the domain. A domain has only one SPF record: edit the existing one instead of creating a second. See also Adding or editing a DNS record and Enabling DKIM, SPF and DMARC.