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What is SPF (Sender Policy Framework)?
A DNS record that lists which servers are allowed to send email for a domain.
How it works in practice
When a message arrives, the receiving server looks up the SPF record of the domain in the envelope sender (the Return-Path) and checks whether the connecting IP is on the list. The result is pass, fail, softfail or neutral. SPF is limited to 10 DNS lookups per check, so every include for a sending service counts against it.
The common mistake
Adding a new sending tool as another include without counting lookups, until the record passes 10 and SPF fails with a permerror for every message.
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