Maybe https://www.mail-tester.com/ would help. Also https://mxtoolbox.com/diagnostic.aspx .

If neither of these tools highlight any issues it's pretty strange indeed.

First step when getting an ip for a server that will be a mail server is to check if the ip is not already blacklisted.

You can always get it unlisted.

After that don't start to send hundred of email by day. You need to build a reputation for your domain and ip.

As the parent comment says, set up directly spf, dkim and dmarc (also arc if you can). Rspamd can help you do that.

I've been running a personal mail server for 5 years with simply following those rules.

If I ran an evil email monopoly, I would systematically drop all email from say _half_ of the email servers out there. This way, nobody would be able to make a case that we're abusing our monopoly position, but at the same time running an email server is hard and the internet is filled with wildly varying experience reports (which is the best FUD there is).

This way, fewer people start email servers and, therefore, fewer potential competitors grow up.

Obviously I have no way to tell whether Google really does this, but if they would, the result would look exactly like this HN thread.

I'm probably totally wrong here, but note that there is absolutely no incentive for the large email providers to try to fix this mess and make email the free distributed network it once was.