First and foremost you want to make sure SPF, DKIM, and DMARC are valid from the sender's mail server https://www.smartertools.com/blog/2019/04/09-understandin

First and foremost you want to make sure SPF, DKIM, and DMARC are valid from the sender's mail server https://www.smartertools.com/blog/2019/04/09-understanding-spf-dkim-dmarc https://www.smartertools.com/blog/2019/04/09-understanding-s... Then, email filtering companies use things like IP Address of the sender, keywords, and NLP, and other tools to determine if the email has spam and assign it some kind of score to it.