You can check firewall setting in your droplet and allow incoming or outgoing port 25,465,587 from all IPs.

Your_Droplet -> Networking -> Firewalls

Hello, all

We have a tutorial on how to set up a Firewall with UFW for Ubuntu 18.04/20.04

UFW, or Uncomplicated Firewall, is a simplified firewall management interface that hides the complexity of lower-level packet filtering technologies such as iptables and nftables. If you’re looking to get started securing your network, and you’re not sure which tool to use, UFW may be the right choice for you.

You can check the tutorial here:

https://www.digitalocean.com/community/tutorials/how-to-set-up-a-firewall-with-ufw-on-ubuntu-20-04

Regards,
Alex