There’s one of two problems here:
- mailgun is dropping your incoming connection attempts to their port 587 (we don’t know if they ever do this)
- Digital Ocean is dropping your outgoing connections to port 587 (we KNOW this happens)
Given that we know that that (2) actually happens, it’s reasonable to assume it’s happening in this case and there are two ways to solve it:
- Contact Digital Ocean and ask them to remove the outbound port 587 block
- Switch to using port 2525 as @pfaffman suggests - Mailgun accepts mail on port 2525 for exactly this reason.