But we haven't started using regular expressions yet...!?
I was fortunate enough that my solution was to host my own mail server 20+ years ago and create a separate email address per relationship with a company, so I can tell the moment some 3rd party has been comprimised when I receive spam on a specific address. My personal spam has been minimal over time.
If for example moc.elgoog@mydomain.com gets spam - I know they're compromised or have sold me out.
Yes gmail has had something similar using the + character, but most people don't know about/make use of this and still abdicate spam filtering to things they don't understand like bayesian algorithms which suffer from false positives. (Have you checked your spam folder for our very important message...?)
Email has never been secure and despite modern updates, I still don't consider it as such. Then again I don't have much to worry about, so I'm ambivalent most of the time. That said, special 'fuck you' shoutouts to Ticketek for being compromised and their general ineptitude and shitfuckery in so many ways... It took them 2 months to respond to an issue I raised with them only to ask whether it was still an issue... (yes, it still is).
Unfortunately I don't know if you could easily manage to convince majority email providers you're legitimite with a new domain in this day and age - I suspect its now a major hurdle to overcome as I've read often enough of mail bouncing because "we've never heard of you until now, so we don't trust you" - which makes communicating with the majority of the world via email almost impossible to build up the trust level you're considered legitimite and that's despite all this extra DMARC, DKIM, and SPF and SSL/TLS supposed safeguards which have appeared over time and I've had to comply with.
Security as an afterthought means its still probably never going to be secure. I've always considered email the equivalent of transmitting plaintext and have always treated it as such. This has led to some pretty difficult situations where I don't email important stuff to a 3rd party just because they expect it and everyone else does it.