This article describes the maximum supported mailbox for FortiMail.
1) Although the datasheet specifies the maximum number of mailbox supported (For example, a FortiMail 2000E appliance supports a maximum of 2000 mailbox, however, this maximum number is referring to the maximum number of mailbox entry supported by the user import function (LDAP users imported via CSV, and the maximum number includes local users as well).
2) It is possible for the FortiMail to support more than the maximum number of mailboxes specified in the datasheet for LDAP users.
3) Taking FortiMail-2000E as an example, after it has been imported 2000 users to the FortiMail appliance, no new users can be imported to the FortiMail anymore, however, a user mailbox will be created under the Users Preference tab once a new LDAP user has successfully authenticated (Either through webmail or email client), thus making it possible for the FortiMail to exceed the specified number of mailbox in the datasheet.
4) This number is however, not unlimited as it depends on the usable HDD space as each user will be allocated a certain amount of disk space, and other constraints such as the number of new and concurrent SMTP(S)/HTTP(S)/IMAP(S)/POP3(S)/LDAP(S) connections, and another constraint is with the number of content filtering and DLP policies that are required.
It would be recommended to engage the local reseller or Fortinet sales engineer to architect the right solution.