Possible Clues:

• They get stamped on
• There must be hundreds in such correspondence
• Characters in literature
• A to Z, e.g.
• They authorise in writing
• Does not drink in the reels in a mess in the mail
• A, B, C, D etc
• It takes 34 across to make one post
• They won't prevent you from reading them
• Contents of this puzzle's theme