Why have you never been able to do it? I set up a full mail system years ago on a Xen/Linux VPS with stuff like Postfix, maildrop, Courier IMAP, a custom set of MySQL tables for aliases and such, and at one point migrated my TLS from CACert to LetsEncrypt. I enjoyed some aspects of the huge pain in the ass that all of that was, and having it work nicely was great. Spinning up a new email alias was easy and free, so I created a new one for damn near every site I interacted with, which later turned into a form of lock in having to continue running my server.

The continual server maintenance was a pain in the ass, requiring me to remember in substantial detail how it all worked so that I could appropriately integrate new things I had to learn like SPF and DMARC. I’m glad to have had some detailed sysadmin experience, but I was so glad in the end to finally migrate away from all that and just pay Fastmail instead.

I still have nearly the same flexibility with Fastmail and my custom domains, but they’re the ones that need to do all the maintenance. I can’t scale across unlimited domains for the same zero marginal cost, but I can make it work for a reasonable price with a few domains and scale arbitrarily within that. I’m sure there are other hosts out there that do a similarly good job, and Fastmail hasn’t been without its own troubles, but it’s been a net win for me.

I don’t recommend running your own server. I won’t do it again. I do recommend building an army of custom aliases all at your own custom domain(s).