I have several domains that I use for email aliases and I no longer need all of them. I’m worried if I let one expire and someone else purchases the domain, they will be able to set up a catch-all email address and intercept any emails that I don’t specifically migrate accounts/unsubscribe from newsletters. What are my best options for preventing this?
Keep paying the 20€ yearly seems to be the only way. Keep it shut down (remove dns entries) so it will eventually disappear from lists as mails can’t be delivered anymore. Relying on that to happen is not a 100% airtight solution.
If possible it might be better to set the mail server to deliver to a special inbox but report the delivery as failed to the sender.
That way you get visibility on what might have gotten in there, while also getting marked as dead for anyone trying to send to it.
If nothing came in after a year or two, it’s probably safe to throw away.