This setting means it initially fully downloads full copies of the last 5 days, but it will only download headers for all the other days. Meaning you still see all emails in Message List, so you can still access them, but those that are just headers will need to access server inorder to download to a temp cache inorder to read them. As the days continue those emails previously fully downloaded are still fully downloaded, so you need to compact the folders so they remove the fully downloaded emails and replace with just headers. This means you only keep 5 days worth of fully downloaded mail, thus reducing the amount of space on computer used for fully downloaded copies. It also depends what you have selected in the 'Message sychronising' 'Advanced' button setting. EG: If you have selected all your folders to download full copies, then the above settings will download the 5 days and headers for others. If you have any folders not selected to download full copies then you will never get fully downloaded copies for the emails in those folders - just headers get downloaded. At this point you need to check what folders are set up to download full copies. Then compact all folders so they remove the fully downloaded copies and replace those older than 5 days with just headers. Restart Thunderbird to ensure everything is properly saved in files.