I’ve been a Fastmail user for years, having left Gmail. It works great and have nothing be but praise for them. I use my own domain with them so if I decide to leave it’s not an issue worrying about updating people with my new email.
So does mailbox do from OP. Just after some time, depending on which package you had. Eg after your light package expired, the address is free for reregistration after 90 days.