Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to follow up on a delayed insurance claim, use this factual base: include the product or service, order number, dates, price, fault, prior contact, and supporting record.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Request a specific lawful remedy and give a reasonable response date before formal escalation. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not exaggerate, threaten staff, publish private details, or demand compensation unrelated to the loss. Use payment, regulator, insurance, police, data protection, or legal channels for fraud, danger, or unresolved rights. You do not need the other person to agree with a boundary for you to follow it consistently.