Mail distribution is only visible to those who are authorised to distribute mail. «Mail distribution» is the standard authorisation group for this purpose and is given the relevant permissions.
However, it is also possible to create a new authorisation group and authorise this group to distribute mail. This will allow all employees in the group to see the section «Mail distribution».
This section is upstream, as this is where the mail arrives but has yet to be distributed. Once the mail has been successfully distributed, it will land in the Mailbox or Invoices section (if it’s an invoice or a reminder). Only those to which the document has been assigned will effectively see it.