Outbound Glossary
SPF
A public list of who may send mail for your domain. Getting it wrong is the cheapest way to land every campaign in spam.
SPF stands for Sender Policy Framework. It is one line of text published in your domain's DNS that names the servers allowed to send email on your behalf. A receiving mailbox looks at that line, compares it with where the message actually came from, and uses the answer as one signal in deciding whether to trust you.
The practical traps are boring and common. You get ten DNS lookups and no more, so a domain that has collected a provider per campaign over the years silently breaks the whole record. And SPF checks the envelope sender, not the from address a human reads, so a message can pass SPF and still be a forgery. That is why it is never used alone.