I've had real trouble with digital ocean as they just don't want to get involved in helping email providers at all. They don't want to do anything with managing IP reputation.
I know. I think there is a study, that showed digital ocean is the provider that sends the most spam emails, and just doesn’t care. As self defense, some providers just block their ips because of that. And if you try to unblock such an ip, they require you to do a lot of erfrort. You can skip it, if you get an IP from a provider with good reputation. AWS for example doesn’t kid around if you send spam. You need a special permission in the first place to even be able to use port 25 outgoing.
