Because of Law 183/2011 Article 15, starting from 01/01/2012, certificates to be produced for public administrations or public services operators (for example: Poste, Permit of stay, Study visa, Embassy, Consulate…) can not be issued nor accepted.
Interested parties, in the cases above, can only produce self-certifications, which have to be accepted, according to the mentioned law.
Those who require certificates to be produced to entities other than public administrations or public service operators, have to ask them to the central PhD Secretariat: http://www.dottorato.polimi.it/en/during-your-phd/forms-and-certificates/index.html
Be careful, in these certificates with stamps it’s written: “This certificate is not valid for public administrations and public services operators”.