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 pending legal review, use this factual base: summarize the deliverable, version, requested input, owner, and effect on the schedule.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Give a small set of options and explain which default plan applies if the real deadline passes. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not bury a scope or price change, threaten the relationship, or pretend silence means approval. Use the contract and documented escalation route for legal, payment, privacy, or security decisions. You do not need the other person to agree with a boundary for you to follow it consistently.