00104: Certificate from an untrusted authority
What it means
SdI code 00104 means the certification authority that issued the signing certificate is not on the list of trusted authorities.
The usual cause
The certificate does not come from a qualified certification authority that SdI recognises.
How to fix it
Sign the file with a qualified certificate from a trusted certification authority and send it under a new name.
The code as published
Read the official document . Source: Elenco dei controlli effettuati sul file fattura del Sistema di Interscambio, version 2.0, published by Agenzia delle Entrate, fatturapa.gov.it, read on 8 November 2026.
Klarbon explains each code in its own words; the official wording is in the document linked above.