Running your own email is increasingly an artisanal choice, not a practical one
Over on Twitter, I said something that's been on my mind for a while, and is a bitter thing for me to accept:
Gloomy sysadmin take: it's no longer possible for ordinary organizations to operate a quality email infrastructure themselves. You can run a little artisanal one and feel happy about it, but it will not at all measure up to the quality of systems run by eg Google and Microsoft.
This is not directly about the big providers making it harder and harder to send them email, although that doesn't help. It's because a quality modern email environment is big, complex, and takes a lot of work to create and keep running.
To be clear, you absolutely can still run your own email infrastructure, getting email delivered to you, filtering incoming spam, sending email (with DMARC signatures and other modern email practices), providing IMAP access, and even run your own webmail setup. You can even do this with all open source software. But the email environment you get this way is increasingly what I called an artisanal one. It's cute, decent enough, and hand-crafted, but it doesn't measure up in usability, features, and performance to the email infrastructure that is run by big providers. Your IMAP access might be as good as theirs, but things like your webmail, your spam filtering, and almost certainly your general security will not be as good as they have.
In short, if you run your own email infrastructure, it will not be up to the general quality you could get from outsourcing to big providers (they can't really be called specialists). And you cannot fix this by trying harder, nor with the magical right choice of open source software, nor with the magical right choice of commercial software. Entirely "on premise" email is now an inferior thing for almost everyone.
Once upon a time this definitely was not the case. The big provider offerings were inflexible, limited, and often below the level of service and quality that you could achieve, except in limited areas like GMail's webmail interface. Although it's a bitter pill for someone like me to swallow, this is no longer the way things are. The big providers are crushing the field through the sheer amount of resources they can throw at problems, including and especially problems that matter to users like security and spam filtering. Getting even close to what they provide increasing requires climbing on board with things much like them (for example, a third party cloud service to spam-filter your incoming email). Especially, open source can't compete on features like webmail and performance in things like spam filtering.
(And let's not even talk about additional services like organizational calendaring.)
Artisanal email systems can still have some virtues, just like other artisanal things do. You can be more responsive to the special desires of your users and implement features they want (to some extent) and have high limits on things like mailbox space, and you're independent; some people will value that enough. Also, there will probably always be some people who use run and use independent email systems, just as there are people who use IRC.
(Our email system certainly supports a variety of features that the big institutional email doesn't and probably never will. But there are also things we just can't feasibly do, like add features to the webmail system we use.)
In the old days it made sense for a lot of organizations to run their own email systems. Often they had no choice about it, because there were no real alternatives that could meet their requirements at acceptable cost. But those days have been fading for years and are probably gone for most organizations.
(For individuals and perhaps very small organizations the picture is murkier, because you have more to lose to the capricious and random actions of big providers and fewer remedies available when they abruptly close your account.)
PS: The other issue with artisanal email systems is that the organization needs people who can run them, and those people have to spend time on email; generally the better and more artisanal a system you want, the more time it takes.