Status: 5.0.0 (box that receives only Certified Mail)

I hope some from Italy can help us with this problem: Everytime we send an email to the University of Bologna we receive this error message Reporting-MTA: mx.cert.example.com Final-Recipient: someusr@pec.example.net Action: failed Status: 5.0.0 (casella che riceve solo Posta Certificata)
We purchased an S/MIME certificate thinking that this may solve the problem but it didn't, so I am wondering what kind of certificate do I need to send emails to that server.