Mostly true (server can be home but using the ISP network directly probably won't work)
You can't successfully use an email server on a (cloud) VPS.
Bullshit
You can't successfully use an email server on a bare metal machine in your own datacenter.
Bullshit
As such, it is my distinct displeasure to declare the death of SMTP. The protocol is no longer usable. And as we can see, this devolution occurred organically.
Sure, you can run one, good luck getting even a halfway decent delivery rate to mailboxes at any major mail provider. Even if they never receive a spam message from your server, your server is an "unknown" which counts against you. And if one person in your small company of 10 or 100 or even 1000 people gets their e-mail hacked and sends spam? Prepare for the rest of them to get punished for it. Running an SMTP server is a nightmare which is why, over time, more and more of the economy has just shifted their SMTP servers to organizations who professionally run SMTP servers instead of having their own.
Set up dkim/SPF properly, make sure the ip you plan to use is clean before you start, sign up for MXtoolbox blacklist alerts and if you get on a blacklist (doesn't happen often if you do a bare minimum of proactive security), you request removal. It's really not hard.
This works, too. It's actually common that your dmarc-entry needs some time to be accepted everywhere. Wait a few days more and your mails don't hit the spam folder on google and outlook.
I've run into one issue in my time on a weird self hosted platform. The DMV said "oh. Let me call IT and get that unblocked" and then they did and it was done. All other times have been fine
I work as a Sysadmin for a web host who sells VPS's. I've helped many people setup domains on their server to cover SPF, DKIM and DMARC passes on a daily basis. Most use these for personal or business level mail delivery without issue.
Are there hurdles to overcome? Sure. But it's not exactly hard as long as you have a IP that's isnt a poor reputation (which as an ISP we help delist and improve). But it's not impossible.
Its more "convenient" to use a third party mail provider just as Office365 since you pass on all that setup and responsibility onto their framework, but it's not hard to setup a decent level of mail service yourself.
You're spot on, and even smaller ISPs routinely get blocked by larger hosters (anyone who doubts this, please look around for the many stories along the lines of "gmail silently drops my email")
Residential IP blocks are scored much higher and given a negative trust from the start - not surprising since that's where much of the world's spam comes from through compromised computers, routers etc.
I'm going to add "bullshit" to the first. I've gone 2 decades running a few email domains on my home servers, on 3 different ISPs. Its not rocket surgery.
I've been running one with a dozen or more users on bare metal at home for the last two years. A little bit of spam but otherwise fine. No deliverability issues or anything.
And for sure your home isp has all the email ports blocked upstream.
With all that being said, to call SMTP dead is wildly insane. I do figure it will die someday though. Probably around the same time of universal IPV6 adoption during the year of the linux desktop.
My ISP doesn't. It an electric company that offers fiber, so not your typical telecommunications company. Still though, not a single blocked port.
On topic, I tried an email server and it is too much of a pain in the ass IMHO, without the requisite training and experience, but certainly not impossible.
My most recent ISP does CGNAT. They don't hide it, it's mentioned in their support pages. A quick email is all it takes to switch you over to an open address though.
Anyway I've got a $5/mo server with akami that looks after my email and it's associated domain.
It took about three hours of following a guide to set up DMARC and etc etc and it works unobtrusively, and is about ten times faster than my old ISP IMAP account that I had for about twenty years.
There is a more recent one that uses a shell script to install all the bits and pieces but I prefer to do it myself so I've got at least some idea of how all the pieces work.
Yeah. I've had zero problems hosting my mail on a bare metalachine in a datacenter. They arrive just like they should, plus it's just so freeing to host it yourself.