Archive folders work by keeping older messages in separate folders, so they are available, but not necessarily in sight. Ordinarily each account has its own Archive folders and stores these alongside your working folders, so it doesn't necessarily mean that you'll be recovering server storage space. If you need to remove messages from the server, or want an offline store independent of any servers, you can move an archive to Local Folders, which means it will be stored on your hard disk and can then be included in regular backup procedure. The settings for archives, under Account Settings, allow you to preserve the original folder structure, so your filling system can be preserved in the archive.