How to mass delete

Is it possible to mass delete old messages, read or un-read?

Do you mean a specific function that will delete any email older than X days (or, indeed, doing so on an ongoing basis)? I don’t think such a thing exists in MS. However, you could do it manually by clicking on the first email you want to delete in a view, then scrolling to the bottom and shift-clicking the last one, and hitting Delete.