Key takeaway:<p><i>"The most logical explanation is that Digital Ocean is a spam-sender's and attacker's playgroup. It is cheap and droplets can be used to immediately send out mail. Bad actors no doubt constantly generate new accounts and rent servers for their nefarious purposes, which leads Digital Ocean IP addresses to become increasingly polluted and worthless for any use involving email."</i><p>This is exactly correct, people set up phishing sites, botnet C&C servers, spam senders, web scrapers, torrent seeders, etc. I regularly ban anywhere from 5 to 50 Digital Ocean IPs every day trying to various evil things (from click fraud to dark hat SEO). What is unclear is what, if any, motivation Digital Ocean has for preventing this. Presumably they are getting paid for the droplet time. Is it too far a stretch to compare it to opening a shop in a sleezy crime-ridden neighborhood because the 'rent was cheap' and then complain that nobody comes to your shop? Some providers spend time and resources policing that stuff, and that costs them money, which is reflected in their hosting costs. Others don't spend any money on that and it too is reflected in their hosting costs :-).<p>Now not to be a complete downer here, you can buy your own IP addresses from ARIN and get them routed to your droplet. The world will be easier for you. But it is going to increase the cost of doing business.
This is not unusual in what is essentially 'shared' IP space within cloud providers. It's easy for spammers to fire up a $5 droplet, send out a ton of spam, and disappear. Rackspace similarly recommends against sending from their cloud IP ranges.
Are you using SPF and DKIM? Those proof that the emails are verified by the DNS of the sender's domain. Maybe IP based blacklists are more likely ignored since providers could rather block based on domains than. Both technologies are at least supported by Google Apps.