Mail::Box

Many of the e-mail modules use MIME::Parser, which reads e-mails from files into Mail::Internet objects. Each such Mail::Internet object is a single simplified message, and you need MIME::Entity (which extends Mail::Internet) to have access to message-parts, decoding, and basic actions as replying.

One of the more developed modules which can handle sets of such messages is the Mail::Folder. It is able to handle various kinds of mail folders. As you can see, differences are purely stored in the folder objects, and not stored in the messages, because each type of folder only uses MIME::Entitys.