When transparent prepopulation is enabled, Microsoft Exchange connections between the client and server remain active even after the Outlook client shuts down. During this period, email data continues to transfer between the Exchange Server and the client-side SteelHead. As a result, when the user logs back into Outlook, the data is already prepopulated locally, delivering LAN-like performance.

SteelHead enables this by creating virtual Messaging Application Programming Interface (MAPI) connections to the Exchange Server for clients that are offline. When the client-side SteelHead detects that Outlook has shut down, it triggers these virtual connections and begins pulling email data from the Exchange Server.

Importantly, MAPI prepopulation does not consume additional Client Access Licenses (CALs). SteelHead simply keeps an existing, authenticated MAPI connection open after the client shuts down. It does not store or use any user credentials during this process.

The client-side SteelHead manages MAPI v2 prepopulation. This version supports a higher number of prepopulated sessions and leverages an accelerated MAPI read-ahead feature for improved performance.

If a user opens a new Outlook session, the MAPI prepopulation session automatically ends. However, if the user starts Outlook in a different location—connected to a different SteelHead than the one handling the prepopulation—the session doesn't end immediately but eventually times out based on the MAPI configuration.

MAPI transparent prepopulation is not supported when using Outlook Anywhere connections.