Greetings!

In this case it sounds like your options may be limited on this side. If inbound/outbound traffic from your droplet is working, but only failing to one end-point, there are only two scenarios that are highly likely:

1. The server you are trying to reach has been blocked in your outbound firewall. This may help you to manage your firewall: https://www.howtogeek.com/177621/the-beginners-guide-to-iptables-the-linux-firewall/

2. You have been blocked by the admin of the other server. In this case, you may need to reach out to them and ask why.

Jarland