For Fastmail, I could not quickly find their policy.

However, I do occasionally receive some emails on my fastmail user address which are not my own, which may indicate that Fastmail user addresses become available after unsubscribing.

The emails are from Steam, with 2FA login codes; I assume they only send those with prior email verification. With only the email address it is apparently not possible to contact Steam and tell them this is not my account (you need to login to contact support, or know a username). The danger here is that many websites (not just Steam) assume an email address is "forever", and the same address can not be used by anybody else.