This feature has been disabled for your inbox in the most recent version of Smartermail. There are current requests for them to re-enable this feature but there is not a timeline for this to happen or even if this feature will be re-enabled.

It is important to realize that it is NOT loading all of your messages! It is still being "paged," but the paging is being done in such a way that it is transparent to the end user. Effectively, it just gets a new page of messages as you scroll down the list while using significantly less bandwidth, per page, than the previous method.