I self host my email, and I have one mailbox, but countless addresses. Everything that needs an email address, has its dedicated one. Not because of security considerations (if someone would get into any of my aliases, I’d be fucked either way), but because I find it easier to filter and manage.
Like,if I get an email to randomwebshop@, and it hasno relation to said place, I will know that they either sold my data, or were compromised. I can then route it to /dev/null, and then everyone who tries to spam that address will be gone from my inbox.
It also makes it easier to tag mail, because I tag based on a property that I control. No reliance on sender, subject, list id or anything that the sender controls.
Currently using postfix + dovecot + rspamd on Debian, but will be migrating to NixOS-mailserver (mostly because I am migrating to NixOS anyway; it’s the exact same stack under the hood, though).
Regarding self-hosting dying: yes and no. I use a relay for some of my outgoing mail, because I have to communicate with people behind allowlists, and I can’t afford to get myself on one. I do not send much mail, so I comfortably fit into the free plan of my relay of choice (smtp2go). Other than a handful of recipients, I have had no trouble sending email anywhere, and I have much more control over what I receive and how by self-hosting. Even if I had to use a relay for most of my outgoing mail, I’d still self-host my e-mail, because it gives me a whole lot more control and privacy. With that said, way back when I started self-hosting, I also had to use a relay for some recipients, for the exact same reason: them using allow-lists. Back then it was my university, now it’s my kids’ school (a curious coincidence, I guess). There were always hosts that played a different game. Sure, they’ve concentrated into Google and Microsoft by now, but I can still send e-mail into those systems, even if through a relay, so self-hosting is still possible, and still gives you plenty of benefits.
I’ve been self-hosting my email for the past… almost 30 years. Today, I think it is easier to do so than 30 years ago. There’s more to set up, but those are well documented, and with solutions like nixos-mailserver, mostly automated away. But the tools got better too! My setup catches a lot more spam now than it did a few decades ago, using a fraction of the resources, and tweaking my spam filters and other properties of the setup are considerably easier too.