Multiple sources and over 10 years of email server shenanigans. IP reputation partially due to low mail volume is a very well known issue for mail servers and DMARC and DKIM are definitely not enough. Here is one article that covers it that I quickly pulled up: https://mailtrap.io/blog/email-ip-reputation/

Mail deliver-ability has been discussed multiple times here on hackernews with the poor state of the postmaster tools being explicitly mentioned in the articles. For example Google is eating our mail: https://news.ycombinator.com/item?id=19756125

> Most of the Postmaster Tools dashboards will only display data when there’s a sizable daily volume of email traffic (up to the order of hundreds) coming from your Authentication Domains and/or certain other conditions, in place to prevent abuse.

https://support.google.com/mail/answer/9983020?hl=en_#zippy=...

Postmaster tool links for additional FAQ's:

https://www.gmail.com/postmaster/

https://sendersupport.olc.protection.outlook.com/pm/