I was using s small British provider, and almost by accident discovered at some point in time that emails I sent to gmail addresses in the prior 2 months had been marked as spam, unless they were a reply something sent to me (so it wasn’t true that everything got lost - only stuff I originated).
I gave up and moved to FastMail because I don’t have the time to deal with this (and solving it once is no guarantee it won’t happen again)
So, email is federated but effectively not decentralized - unless you use one of the central services, you are at risk from being obliviously blackholed at any second (that happens to large providers too, but there’s safety in numbers - it will be discovered sooner by someone)