Furthermore, DigitalOcean silently drops all SMTP traffic on IPv6, which can be really confusing since you'll simply timeout and it isn't mentioned in the documentation. They do that because they implemented IPv6 against all standard industry practices (which were helpfully gathered in a RFC) and within hours of launching IPv6 the few /64s from which they were allocating customers /124s to landed on blacklists.
It's been 5 years since and they still have no plans to do IPv6 properly and allocate /64s [0].
This is the main reason I switched to Linode, who will happily allocate you a IPv6 range (/116, /64, /56) that can be rerouted between VPSes with a simple ticket (takes <1 hour).