Earlier quoted context omitted.
"IPv6 support 1. Took forever to implement, and the timetable broke promises to customers. 2. Inferior. Digital Ocean still won't give you a /64 per standards." Far more egregious is that they silently drop port 25 on IPv6. This means that enabling IPv6 will cause mail problems for some destinations (destinations that support IPv6, like Google). When asked they say it's because a /64 is too much address space in the…
I can't talk about port 25, but we have 15 mailservers hosted on $5/per pop with DigitalOcean and with properly setup rDNS and SPF including our IPv6 address, we don't see any problems with deliveribility to gmail via subscription port. That is on circa 600,000 emails per month sent to gmail.
You are using your users' gmail credentials? Or are you simply delivering over 587 the same as you would over 25?