DigitalOcean definitely does have a terrible reputation (for good reason). IIUC they used to let anyone send mail and I don't think their current process of requesting unblocking is too strong of a prevention against spamr either. However ideally this would only hurt "first contact" for a domain assuming it is authenticated. In practice some providers will barely consider domain reputation even after a long history.

There was a study somewhere once that concluded that digital ocean is the biggest single source of spam emails. And that the are responsible for around >20% of spam emails worldwide.

I noticed that some providers completely block digital ocean IPs on their firewalls. So no way to get unblocked.

The only thing I found that seemed to help with Microsoft is for the recipient to find the mail in the spam box and reply to it.