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 reconnect with a long lost cousin, use this factual base: identify the relationship, last meaningful contact, and any conflict or uncertainty that should be acknowledged.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Offer one limited next step and make no response or slower contact an acceptable outcome. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not recruit relatives, use children, invoke illness, or attach an inheritance issue to emotional pressure. Respect court orders and safety plans, and use a neutral professional route for abuse, estate, or custody disputes. You do not need the other person to agree with a boundary for you to follow it consistently.