It's not generally true unless you're a Capital-E grade Enterprise customer running a single server as a side component of a much larger Internet presence.

Finding connectivity that's not already blacklisted by the GOOG-MSFT email protection racket and will allow individual servers to send outgoing SMTP is basically impossible for sub-enterprise scale organizations. The major cloud services block outbound SMTP for non-Enterprise customers and minor cloud services will already be blacklisted.

Getting your mail delivered by GOOG-MSFT also means that you need to have matching forward and reverse DNS, and ideally you should have your own ASN that has never been associated with any addresses that have ever been accused of sending spam. This is an impossible hurdle except for large organizations.

The entire email ecosystem is stacked for force smaller players to pay a head tax either directly to GOOG-MSFT, in the form of using their mail services and paying extortionate per-user mailbox costs, or by using paid mail relay services.

So long as you do not also send spam, or send mail that someone might think is spam. I'm a mail admin and can see my users's spam reports. Lots of those messages are authenticated and there's actually not that much malicious stuff being delivered in the first place. It is just that the messages aren't wanted.

Once your messages start to get reported, the sender's reputation goes down until its spamscore qualifies for the filter.

It still possible to send mail from a small mail server, but there no grantee at all that messages will not end up in a spam folder. Most likely Gmail uses some ML based spam filter which has relatively high rate of false positives. For big senders they can do something manually, but they don't care about smaller ones. Gmail is too big and have little incentives to improve they spam filter (until users will start to leave, which will not happen anytime soon).

It's mostly true.