The window above is mostly summary. The other part is the mailbox of letters. It's in the attachment in the lower left hand corner of the window. Click on the first, second, or last letter to see its contents. Digest are sent out by mailing lists.
MIME digests are very convenient.
You can save an individual letter in the digest just the way you would save any piece of e-mail. If you want to reply to an individual letter, you reply the way you would to any other letter. All this assumes that you have a modern e-mail program. If you are using Eudora Windows, forget it. There are no individual letters to save, no individual letters to reply to.