In most deployments of MailStore we recommend your journalling job is configured to archive a selection of journalled or 'copied' emails for all your users. This type of job is designed to interrogate each message that it archives and look for headers that it can match to decide which user's archive it should store the message under. In a perfect installation where all of your MailStore users have been setup correctly, every journalled message should find the correct corresponding users archive. But it is common to overlook some addresses and you may find email appearing in the general 'Unknown e-mail archive' instead.