Namespaces of free users expire 1 month after they are created. When a namespace expires, all it's received emails are deleted and the subdomain becomes available for other users.
At any point before or after a namespace expires, you as a free user can add a new random namespace. If you've reached the namespace limit, you can instead erase the existing namespace and add a new one. Thereby resetting the expiration time for the new namespace.
Why do namespaces expire?
We allow free users one random subdomain. To prevent most of the short subdomains being taken due to inactive free users, we opt to expire those subdomains, so they can be reused.
How can I keep a namespace from expiring?
Upgrading to any paid plan will remove the expiration time for your current and all future namespaces. Another benefit of upgrading your plan is the ability to create custom named namespaces.