Mail book in the Document Control Procedure

Answer: No, Mail book is not necessary - you just need to keep track of important external documents (e.g. contracts, correspondence with government agencies, etc.) - where they are, and who is responsible for them.

You can do it through Customer Relationship Management software, or Project Management software, or a simple Excel sheet will do. Or you can define in some document that the responsible person for all correspondence with e.g. government bodies is XYZ, and that this person is responsible for archiving all such correspondence.