Free tool
Make a mailto link that opens a ready email.
Set the address, subject and message, then copy a link or an HTML snippet for your website. It runs in your browser: nothing you type is uploaded.
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