TIL

https://www.fcc.gov/reassigned-numbers-database

> For email, the namespace is large.

> As far as I know personal email providers don’t reuse identifiers.

Email providers vary, their policies can change, and "don't reuse" may only mean "...for a year or few".

Or - if the email address is "@MyDomain.com", you have issues with expired domain name being picked up by less-than-saintly new owners.