Just because you read blog posts titled "DON'T HOST YOUR OWN MAIL", doesn't mean you shouldn't. People are scared of hosting their own mail servers because they a) are told they shouldn't b) don't understand SMTP.
I am all for decentralizing this hyper-centralized internet.
Maybe I'm too old? Has email come around to the point where someone can host their own email and send out thousands of emails without having to deal with a mountain of headaches again? That would be sweet, maybe I can go back to telling clients it's okay to paste a massive list of email into their Outlook's BCC field? It would certainly save them some money. I still to this day hear from clients who send "small" mass emails wondering why their friends at AOL, or <insert your ISP here> didn't get the email.
Sorry but this is based on my direct, first hand experience with hundreds of small businesses doing this exact thing. I don't base my business model off of some random blog post, I base on years of experience and the lessons learned from getting burned.
I run my own smtpd for personal purposes, but if I were going to send outgoing customer-contact mails en masse, I certainly wouldn't do it through my personal perfectly-configured postfix system. Things like mailchimp fill a market niche.
Regardless, hosting your own mail isn't the same thing as using outlook's BCC field.
Tens or hundreds of thousands, maybe not. (And I'm assuming that this isn't email that the recipients are going to consider is spam.)
I think there are two major reasons for this improvement: DKIM basically works, and far fewer recipients are using email accounts provided by crappy ISPs.
Some basic good hygiene and some effort put into gently educating customers on how email works has gone a long way.
My biggest headache today is in receiving mail, not sending it. Because so many other people have followed advice like yours and jumped onto one crappy mail service or another, the originating network for an email is no longer a good signal for whether it's spam or not.
Even if that would be a problem in future, you'd rather use one of SMTP PaaS (there are quite a few of them out there) instead of using fully integrated solution like Mail Chimp. I.e. you can use Mautic and connect it to Sendgrid or something like that. This way you have full ownership of mailing lists and newsletters content.
The hard part isn't actually sending the mail, the hard part is staying abreast of the already long and perpetually growing list of legitimacy signals you need to send to reassure understandably paranoid SMTP clients that they shouldn't mark your newsletter as spam. And you're not done even when the server is set up and configured properly; it doesn't take more than a handful of ignored complaints to start getting added to blacklists.
It's not difficult, per se, but it is a huge time investment for a service that centralized vendors can provide extremely cheaply because of economies of scale.
I'm all for moving toward decentralization too, but we're not going to get there from here if we don't understand and respect the incentives that lead people toward centralization in the first place.