To maintain your active folders at a manageable level so that you can index them once or twice a day in a few minutes, let's say that you move mail that's older than a year old to an +archive folder which may or may not be organized by year. This archive folder would have its own second index which would only need to be indexed when new mail is added to it (once a year).
This report addresses two things. First, the manual should cover how to set up this +archive folder and how to configure the various search engines to look at both your active index and the archive index.
The second item this report addresses is how to keep this archive from affecting MH-E adversely, that is, slow it down. This might be done by adding a configuration option for "ignored" folders like +archive. Commands that would heed this option include `F l' (`mh-list-folders'). The initial folder collection or folder collection for folder completion should also ignore the archive.
Another approach would be to move the archive out of the Mail directory (for example, Mail.archive) and then document how MH-E could browse and view folders in the archive.