One Unknown Recipient.

Dear reader,

One of my collegue’s is trying to open a .pgp file that has been encrypted with Kleopatra. When he tries to decrypt the file, he gets an error stating that the decrypting has failed because of 'One unknown recipient). Do any of you guys (/girls) understand what this error means and how I can fix this? Is it a key-sharing problem?

Regards,

Bakkie.