A missing user on Vaultastic is one who does not have a Vaultastic account, but mails for or from the user were sent to Vaultastic for archival. This can happen if you missed adding the Vaultastic account or the mail was meant for another Vaultastic user but had the missing user id either in the sender or recipient fields.
Mails for missing users are not archived.
The total number of unique users for whom the mail were dropped and the total number of mail dropped in the given period are mentioned in the report.
Use the missing user report to confirm that all the important user accounts are created on Vaultastic. You can also use the Automatic User Creation feature to create the users as soon as the mails hit the Vaultastic servers.
The CSV attached gives a list of Vaultastic accounts for whom mail were dropped a day prior to the generation of the report.
You can subscribe to receive the active users report in your mailbox.