I had DO start blocking 587 in March as well. When I contacted them, pretty upset, they maintained that they did not expressly intend to block my host. I had done some experimentation with postfix (and netcat) and demonstrated I could not reach out on 587 on any of the three ip addresses on the host, not even on their internal subnet between droplets in the Team. Their tech then “reset” my droplet’s networking, which required me to power off, then on the droplet to restore access.
It sounds like they implemented a “global” block in their virtual infrastructure w/o bothering to check who was using SMTP. This is pretty lazy. They could readily have determined if a droplet was using a dedicated commercial relay, i.e. Mailgun, SMTP2GO, etc. and exempted them on a 1st pass. But there is no excuse for not communicating the plan to start filtering / blocking 587 to customers.
On the upside, I found MailerSend (sister co to MailerLite) and my traffic will fit into their free tier vs the Mailgun Flex plan.