In a mail production system comprising at least one user workstation on which a document is generated, printer means for printing the pages of said document, and folding and insertion means for folding the pages once they have been printed and for inserting them into an envelope, a method is provided in which, in order to control insertion into the envelope of each of the pages of the document, a unique marking code is generated, for each page of said document, on the basis of a predetermined combination of characters on each of said pages.