What SPF does
SPF (Sender Policy Framework) is one TXT record on your domain that lists the servers allowed to send its email: your mailbox provider, and any tool that sends on your behalf. A receiving server compares the sending server with that list.
Without it, or with a broken one, cold email from your domain is more likely to land in spam, and Gmail and Yahoo ask anyone sending 5,000 or more messages a day to their users to have SPF, DKIM and DMARC.
What the checker looks at
One record only: two SPF records on one domain are a permanent error, as if there were none.
The DNS lookups: include, a, mx, ptr, exists and redirect each cost one, nested includes included. Past 10, receivers stop and fail SPF.
The ending: -all fails unlisted senders, ~all soft-fails them (the usual setting), ?all says nothing, and +all lets anyone send as you.
Questions
What should my SPF record look like?
One TXT record at your domain, like v=spf1 include:_spf.google.com ~all for Google Workspace or v=spf1 include:spf.protection.outlook.com ~all for Microsoft 365, with one include per other service that sends as you.
Should I end with ~all or -all?
~all is the usual choice and lets DMARC decide. Move to -all once you are sure every service that sends as you is in the record.
My record has too many lookups. What now?
Remove services you no longer send from, or replace an include with the ip4 ranges it stands for (ask the service). Then check again.
Is SPF enough?
No: pair it with a DKIM key and a DMARC policy. Together they prove your email is yours.
Where Coolie comes in
A domain that passes these checks is the start. Coolie then sends from your own mailboxes and LinkedIn accounts, paced over working hours, with daily caps per account and per action, a pause when someone replies and an unsubscribe line: how Coolie protects deliverability.