What's the endpoint running at `include:spf.example.com` as seen in an SPF record in DNS?
Solution 1:
You don't need to run any software, the "include" directive literally includes the SPF result for the stated domain name.
You simply need to create a SPF record (i.e. a TXT record with a valid SPF syntax) in the DNS zone "spf.example.com"; in that record you will mention all your allowed mail senders using IP addresses, host names or whatever.
When you put include:spf.example.com in another SPF record, the SPF result for spf.example.com will be included.
This all happens using DNS, no other software is involved or required.