SMTP port 25 is blocked on all Droplets for new accounts to prevent spam and other abuses of our platform.
Even if SMTP were available, we strongly recommend against running your own mail server in favor of using a dedicated email deliverability platform. Dedicated email deliverability platforms are better at handling deliverability factors like IP reputation.
To send mail from DigitalOcean, we recommend using SendGrid via the SendGrid Marketplace 1-Click App.