Glossary
DMARC
A policy a domain publishes to tell mail providers what to do with emails that fail its SPF and DKIM checks.
DMARC ties SPF and DKIM to the address you actually see in the "From" line, and lets a domain say what to do with messages that fail: deliver them anyway, send them to spam, or reject them. A DMARC failure on a message claiming to be your bank is one of the clearest signs of impersonation.
Many small domains publish no DMARC policy at all, which makes their address easy to fake. Fishi reports both situations in plain words. Forward a suspicious email as an attachment to check@fishi.email.