I have the same problem. I've been running my own mail server since 2000, and last year my sister's account started being used to send spam (I assume she reused her email credentials in another site that got hacked, and spammers used the credentials to authenticate as her in my SMTP server to send spam).

Google shows that my domain has a bad reputation, but I can't fix it because it doesn't have enough traffic. https://www.mail-tester.com gives me a 10/10 (I have DKIM, DMARC, SPF, everything), but my email still gets flagged as spam by Google.