Hi Gianluigi, I'm not completely sure what your provider is asking for, but the way to connect email service to your WordPress.com domain is by adding your email provider's MX records to your Custom DNS settings. Your email host will need to provide you with the correct MX records for their server, and then entered in the format specified in our guide to adding email.
You may want to send your provider the link above to be sure they understand what is needed from them in order to set up email here on your custom domain.
If you need further help, just let me know.