People keep using my mac.com address to register for various things, including one bank account. For Instagram I had to register all the mac.com/me.com/icloud.

People keep using my mac.com address to register for various things, including one bank account. For Instagram I had to register all the mac.com/me.com/icloud.com variations as new accounts simply to stop people from creating accounts using that address. So, please verify the address. You can decouple it, ie allow someone to go through the flow they're in, but only lock in the transaction (commerce, comment, whatever) once they've verified the address. If they don't verify the address in N hours or days, cancel the transaction.