SPF
A DNS record listing which servers are allowed to send email for your domain.
SPF, Sender Policy Framework, is a line published in your domain’s DNS that names the servers permitted to send on its behalf. A receiving server checks the sending address against that list and treats mail from anywhere else with suspicion.
When you start sending through a new platform, adding its servers to your SPF record is part of the setup. Skipping it is one of the most common reasons a first campaign lands in spam.
SPF alone proves little — it can be satisfied while the visible “From” address is forged. It works together with DKIM and DMARC, not instead of them.