Clients already fetch all emails locally over the years you use the email. You don't have to build the index in one pass. As you read/download each new email, y

Clients already fetch all emails locally over the years you use the email. You don't have to build the index in one pass. As you read/download each new email, you add it to the index segments. The same way you don't have to download the internet to create your searchable browser history.