Pro move: type dmarc:yourdomain.com or blacklist:1.2.3.4 straight into the box — it switches tools for you.
About this tool. SPF is a DNS record listing every service allowed to send email as your domain. Broken or duplicate SPF gets your mail junked. This check validates the record, counts the 10-lookup limit that silently breaks SPF, and names the senders it recognizes.