"The fundamental issue is that SendGrid’s business model depends on making it easy for legitimate businesses to send email at scale."

I disagree with this conclusion, if not only because other email service providers don't have this issue.

It wouldn't surprise me if something was broken with SendGrid's internal infrastructure. I used to be a SendGrid customer until my deliverability started being affected by this issue. SendGrid took weeks to reply to my customer service messages about resolving this, even though I was a paying customer and was renting private IP addresses from them to send mail.

I finally gave up and closed my SendGrid account in July 2021. Despite this, they continued to send me monthly invoices until May 2022. Multiple SendGrid representatives promised that they had resolved the issue, but it wasn't until one CSR added me to SendGrid's global suppression list that they finally stopped.