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 apologize for missing a client call, use this factual base: state what went wrong, which task or person was affected, and what is already known about the impact.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Correct the error, name the owner of each next step, and provide a dependable update time. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not blame workload, hide material risk, or promise a deadline you cannot meet. Use the required manager, HR, security, or compliance route when confidential data or employee safety is involved. You do not need the other person to agree with a boundary for you to follow it consistently.