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 thank a medical team after treatment, use this factual base: separate symptoms, diagnoses, medication, dates, and clinician advice from assumptions or internet research.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Ask for the record, appointment, review, accommodation, or care decision through the correct channel. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not share another person's medical information or treat a message as a substitute for clinical advice. Contact local emergency or urgent medical services for severe, worsening, or immediately dangerous symptoms. You do not need the other person to agree with a boundary for you to follow it consistently.