I realized the truth of this this year, when I launched my own self-promotional website (just a portfolio site) and assiduously configured its email, which passes every single test on the range of spam-rating testers available.
Almost none of the mail gets through.
Now I am researching outsourcing the site's mail to one of the usual suspects: mxroute, FastMail, ProtonMail, Exchange, etc. Perfectly configured mail that no-one gets to read is worthless.
It takes a couple of years, at least, of sending spam-free mail at a rate of at least a dozen a day to establish a reputation. Well, I don't know - it might be worse than that. My domain started sending mail at tha kind of rate back in 2004; deliverability to gmail and especially hotmail was never something I'd bet my house on. Hotmail, in particular, would swallow mail and silently fail to deliver.
Yeah, domain reputation works more or less just like IP reputation for a lot of email providers. Holding onto the domain for a long time and sending some amount of email from it will help it get "known good" scores with the big providers. I would also recommend setting up DMARC reports because I've heard anecdotes (no evidence here!) that some providers give a positive bump to email from domains they can successfully return DMARC results to... and it'll at least make sure you know if you have an SPF/DKIM setup problem, especially if it's somehow a transient or subtle one (I've had this happen with config mistakes where not all outbound email was being handed to opendkim).
Almost none of the mail gets through.
Now I am researching outsourcing the site's mail to one of the usual suspects: mxroute, FastMail, ProtonMail, Exchange, etc. Perfectly configured mail that no-one gets to read is worthless.