Comment by neuronflux

They probably sent it from gmail which would pass the SPF check (google.com and gmail.com have the same SPF). They wouldn't have it signed to pass DKIM, but google doesn't use strict alignment checking so to pass DMARC either SPF or DKIM are acceptable.

    ~ dig _dmarc.google.com txt +short
  "v=DMARC1; p=reject; rua=mailto:[email protected]"