Mozilla has the only implementation at the moment as they are still developing it, but the intention is that any email provider can provide this in the future. They have not written themselves into the spec. In fact they have commented that the more successful this standard is, the less important they become.
Commendable goals, but in my opinion somewhat conflicts with the 'Mozilla' prefix in the name, if they hope to make it a widely adopted standard they need to drop their branding and just call it 'Persona'.
