Er, no, it seems anti-spam. The fact of the matter is, plenty of people use dime-a-dozen hosting to make spam bots. These days, you're better off using a reputable mail service as opposed to rolling your own, unless you have a fairly large company.
It's probably a problem they aren't in a hurry to solve either.
You have to spend a long time to build up a reputation to get places to mark your email as not spam. This is why many providers that allow you to do bulk email through them have multiple IPs and they only let the most behaved on their better reputation IPs.
Google (and basically every other reputable email host _and_ every "independent" email host) are blocking mail from services that host cheap compute power. A lot of Azure and GCP email traffic would likely be blocked, not just Digital Ocean.
If you want to start an email service, use something that isn't a known platform for spammers.