That being said, now I monitor and auto-ban failed authentication attempts to smtp/imap (among others) and running the service is fairly low maintenance.
But the morale of the story is that you are only one weak password from one of your users away from your mail server getting blacklisted as a spam server. So while I think it is fairly easy to run a personal server, running one for a small organisation is another matter.
What ISP should someone choose?
I'm doubtful a default block would work, as that would even penalise the 'big boys' of email when they make basic network changes and piss off existing customers of both sender and receiver... Its easier and logical to conclude something without reputation yet is therefore sending too few mails to be useful to a spammer.
I've had good experiences with smaller ISPs (currently Mythic Beasts). In contrast, OVH was a poor experience.
I find that reputation (beyond the known "block-lists") appears more likely being tracked for the whole AS number, therefore a lot more to do with your "neighbours" than anything else.
What matters most is if the IP address they issue you has been blacklisted for spamming. DigitalOcean is fine but you need to check the IP address before you do the work of building a mail server. Some of their IPs are on a lot of blacklists.
If it's only on a very few you need to look into who's blacklisting it. There are some that offer a way to get delisted and make it easy, there are others that block pretty much every IP address DigitalOcean has (or large ranges of them) and they won't de-list anything within them. Many of those blacklists are managed overseas and not used much in the U.S.
No matter the ISP you should check the IP address they issue for a VPS before you build the email server.
My personal gmail account is full of spam and emails I do want from email lists end up the spam folder randomly.
That's about as easy as it gets but it still requires some work and you need to check the IP address DigitalOcean issues to see if it's blacklisted before you set it up.
Google makes it easy to get whitelisted. Microsoft email services (Hotmail/Outlook) are a pain though. I tried to get through their process but got nowhere. Other services I had to submit a request to get de-listed. So it does take awhile to go through all that.
Still, I prefer that to hitching that wagon to a 3rd party provider like Google, or any other.
Before I set mine up the 1st time I'd been screwed a few times by 3rd party providers. The last one, I can't recall which, but it was either "MailChimp" or whomever bought them, that I'd configured an app to use and almost as soon as I'd released it they announce they'd been acquired and I would have to use the new services APIs, and of course they cost more, and their services were geared towards mass mailing, and that's not what my apps do, and their API sucked for my needs.
It was about 12 years of dealing with 3rd party bullshit that motivated me to set up my own email server.
If you just want to fiddle around with one to get a feel for it Mail-in-a-Box is a good place to get started: https://mailinabox.email/