Koltrix
Free tool
Read email headers without squinting.
Where a message came from, every server it passed through, how long each hop took, and whether SPF, DKIM and DMARC passed. It runs in your browser: the headers are never uploaded anywhere.
In Gmail: open the message, the ⋮ menu, then “Show original”. In Outlook: File → Properties → Internet headers.
Nothing leaves your browser — this runs locally.
Tired of doing this by hand?
Koltrix generates these records for your domain and checks them live while you set it up. 7 days free, no card.
More free tools: all of them · SPF, DKIM and DMARC explained