But when a moderately technical colleague wanted to do the same, I told her to use Mox, she set it up and Gmail doesn't block her either.
So... would you please elaborate?
It was dead-nuts simple in the 1990s: Just learn enough about DNS to put in an MX record that points to an A record, get sendmail working, and have it begin delivering mail. The end. (Open relay? No spam filter? No virus scanning? No nothin'? Yeah, that kind of was the style at the time...)
It's got a lot more steps today, but it's still do-able. Operationally, keeping a mail server online and treated well just takes one or two people to spend a little bit of time occasionally to stay proactively ahead of new expectations and requirements instead of reacting to them after things change.
It also helps if Carla, from marketing, doesn't wake up one day and decide to spam the entire customer list without asking for guidance first. Maybe I should have put some automatic mitigation into place for that, but whatever: We chatted about that and it never happened again.
(Or at least, I find that to be true with smaller companies. Bigger ones obviously may require more elaborate systems to handle more volume and/or provide better uptime. But the requirements of keeping the reputation up are about the same regardless of scale, and that still only takes one or two people to pay attention to things sometimes. [And the only reason two might be required is in case one of them gets hit by a bus.])
I am definitely not being flagged for any actual spam-like behavior. I might send out 40 e-mails a year, and even though it's a "family" domain, I'm the only one who has ever used it, ironically enough, as part of my decade-old effort to de-Google.
I also have my own MTA. No problems with anyone ..... except Microsoft, who (silently) never delivers the mail.