Mail User contains everything related to a single user.
Mail Namespace A single user can contain multiple namespaces.
Mailbox List is used to list/manage a list of mailboxes for a single namespace (1:1 relationship).
Mail Storage is used to access mails in a specific location with a specific mailbox format. Multiple namespaces can point to the same storage. A single namespace may in future (but not currently) point to multiple storages (e.g. a mixed mbox and Maildir directory).
Mailbox is used to access a specific mailbox in a storage.
Mail is used to access a specific mail in a mailbox.