I've run 5 or 6 different mail servers over the past 10 years. Originally before O365 I was an exchange admin, then postfix, iRed, mailcow, mail gun, you name it. Hosted on every cloud provider, even in our colo with part of a private /24 allocation with good reputation (built since 1997, gawdamn). Every sort of header combination, tls setup, and no blacklists. Always 100% alignment, including strict rejection policy (best results even over quarantine).
Does not matter, if you're sending from custom domain not handled through a big name, expect the spam box with Gmail. Yahoo and Outlook are fine, but Gmail is the bane.
I've spent maybe 100 hours of my own over this last year and know what I realized? Nobody cares about email anymore, except for automated account management stuff (login, PW reset). Businesses pay the $3 /mo / seat for fastmail and don't think twice.
But the current trend is toward social chat (discord or Whatsapp) and most the people who own an iPhone just use their apple ID email for everything.
Although I am a fervent supporter of open protocols and believe email (with pgp signing) is an awesome long form communication format... Face it, it's going the way of the fax machine.
However, email has basically evolved into the way you communicate with “systems” and I’m kind of happy about it. Communication with companies outside your network, e-commerce accounts/purchases, communication with government systems, schools, banking, airlines, concerts/events, restaurants, etc. Hell, even RSS is now basically in email — newsletters are growing fast as a medium, not shrinking.
You just book a hotel in Nairobi? It’ll be in your email. No other communication method even comes close for this use case.
Social/chat apps will never unseat this because they’re social. Like nightclubs, the trendy ones come and go. Come back when you’ve set up an interoperable network of virtually every person on earth. Then we’ll talk about email being dead.
Email is the primary way that I stay in touch with my extended family and friends. At least for us, it's very much alive.