MxToolBox Fully Managed Hosted Email services are powered by Zimbra, a linux-based open-sourced competitor to Microsoft Exchange owned by Yahoo! It is a full messaging and collaboration suite which provide a secure, reliable, productive hosted email experience for you and your users.
As each message is delivered to the server, it's meta-data like Sender, Recipient, and Subject are stored into a database for super quick access.
The architecture allows mailboxes and mail servers to scale so very large mailboxes which would choke other systems are not a problem.