Silly question I know. But couldn't help but wonder if my old ISP will allow someone else to use one of my old email addresses or if that address can never be used again...
I would doubt it, usually ISP's assign your email by some variation of your last name. Hell my parents had access to their old bellsouth account for months after they cancelled it and switched to comcast.
When I had left Comcast, my email accounts still worked for at least a year after that, then they go inactive. I don't know how long the wait time is until they are "out for grabs" though.