I run my personal mailserver on a DigitalOcean VPS, but I wouldn't recommend it. I've never had blacklisting issues and email works fine over IPv4, but outbound connections on all mail ports (25, 109, 110, 143, 465, 587, 995; not 993, oddly) are silently dropped over IPv6. They assign each host a IPv6 /124, instead of a /64 or better like other providers, so they block outbound SMTP for spam prevention. The other blocked ports aren't documented and I've had a support ticket open since November about getting them unblocked that looks like it won't be resolved anytime soon.
DO has been great for everything else I hosted there, but they're a bad choice for a mailserver.
DO has been great for everything else I hosted there, but they're a bad choice for a mailserver.