Related Clues:

• Ltrs. in a letter
• Ref. wd.
• Something sent with a ltr.
• Env. attachment
• Abbr. on a business envelope
• Bottom-of-env. header
• Check, perhaps: Abbr.
• It's added to an env.
• Unsolicited MS., perhaps
• Env. add-in
• "Look inside" abbr.
• Stamped return env., for example
• Placed inside: abbr.
• Routing abbr.
• Something inside an env.