I wonder how long they wait to recycle an alias?
Some years ago, I created an alias using one of the fastmail domains.
Later I went to create a subscription to an online magazine.
It said that the email I used was already an account holder.
I thought, maybe I forgot I already subscribed.
So I sent a password reset request.
When I logged in I found another person's details, name address etc
So clearly somebody before me had used this alias to create an account at this site already. They must have then released the alias or deleted their fastmail account.
I thought that was pretty scary and alerted me to not use fastmail domain aliases in the future, in case I am the one that has forgotten to remove those details before I release the alias.