Definition(en): This variant of ZFO file is container for proof of delivery of data mailbox messages provided by Česká pošta, s.p. in three versions: 1. proof of delivery; 2. confirmation of delivery; 3. delivery failure. ZFO file is a ZIP archive file (PKCS #7 Cryptographic Message File) composing of the layout structure using XML elements. The whole structure is always embedded into a digital signature represented by XML elements of the XML Signature standard.