How would you handle making them non-permanent, though? Expiring hotmail addresses have been problematic in many cases, and any unique lookup string will event

How would you handle making them non-permanent, though? Expiring hotmail addresses have been problematic in many cases, and any unique lookup string will eventually be stored somewhere by someone and assumed still valid later on. It's not even solved in full for phone numbers, despite everybody knowing they can expire and be reassigned - since long before our own lifetimes.