This is something you would need to do one your account. If you are using software like UFW open the ports there, or if you are using the DO firewall you can do so via your account control panel.

Here is a great article on DO Cloud Firewalls

https://www.digitalocean.com/community/tutorials/an-introduction-to-digitalocean-cloud-firewalls

Contact DO support - there is a chance that outbound mail traffic is blocked on your account:

https://cloudsupport.digitalocean.com/s/createticket