Glossary
SPF (Sender Policy Framework)
A public list, published by a domain, of the servers allowed to send email on its behalf.
SPF lets the owner of a domain, such as a bank, publish which mail servers are allowed to send its emails. When a message arrives, the receiving provider checks whether it came from one of those servers. If not, the result is a "fail" or "softfail", a sign the sender address may be forged.
SPF alone isn't enough: it checks the hidden envelope address, not always the name you see. That's why it works together with DKIM and DMARC. Fishi reads these results when you forward an email as an attachment to check@fishi.email.