This feature,  introduced with Notes/Domino 8.0, will “clean-up” user mail Inbox folders. In other words, it removes ( not deletes ) documents from the Notes/Domino mail Inbox. The goal is to improve server performance by reducing user mail Inbox size. An Inbox folder with fewer documents performs better because it has fewer documents to index and replicate.

Frequently asked questions are answered in this FAQ. One of the questions is about how the feature works in a mixed environment with 7.x and 8.x mail templates.

The FAQ says that Inbox Maintenace only works with 8.x mail templates. But it is very easy to even enable a 7.x mail template.

The only thing you have to do is to copy the LotusInboxCleanup agent from mail8.ntf to your 7.x template.

The server looks for the existance of the agent and triggers it  if available. Otherwise it will skip databases that do not inherit the agent.

Although the agent is of type scheduled, you do not have to setup a schedule. The agent is invoked by the server.

Settings are on the “Server Tasks -> Administration process” tab of the server document.