Possible Clues:

• Message written in protest
• Protester's medium
• Message addressed to an individual but intended for all to read
• Martin Luther King Jr.'s message from Birmingham Jail, for one
• Slit the envelope; it's not a private communication
• Editorial piece, perhaps
• Exposed, landlord in missive made public
• Post for all to read
• Use knife, perhaps, in widely-publicised protest
• No confidential correspondence from generous landlord