Possible Clues:

• Slit the envelope; it's not a private communication
• Protester's medium
• Martin Luther King Jr.'s message from Birmingham Jail, for one
• Post for all to read
• Use knife, perhaps, in widely-publicised protest
• No confidential correspondence from generous landlord
• No private correspondence to begin: 'Old Impeder'
• Public communication welcoming landlord?
• Means of public protest
• Editorial piece, perhaps