SMTP is disallowed over IPv6 on DO. AFAICT (I may not have this 100% correct), if someone sends out spam email from someone hosted on DigitalOcean, under IPv6, the whole range from emails of all DigitalOcean users are blocked. Whereas under blocking an IP range from a similar user in IPv4, it would not be locked. Since email providers would (or possibly have) block the range of IPv6 IPs associated with DigitalOcean, all SMTP traffic from DigitalOcean is blocked when IPv6 is enabled on a droplet.
However, SMTP is a common protocol used by servers whose focus is *not* primarily as an email server. One example is the popular open-source project Mastodon, and I wrote a blog about my experience here: https://medium.com/@uistephen/part-2-creating-your-own-mastodon-instance-digitalocean-and-nginx-55686b751efe
As I say in there, I experienced issues where my emails were failing to send from my application and it was not immediately clear why that was.
For that reason, my request is that you warn people enabling IPv6 on a droplet that SMTP will not work as expected. I did come across some workarounds but they didn't work for me. If I had a warning (such as a tooltip (?)) next to the IPv6 checkbox that warned me that IPv6 would not work correctly if I enabled it, would have saved me a fair amount of time and confusion. Thanks.