You may need to contact DigitalOcean support - they don’t make it obvious to a customer AFAIK if SMTP is blocked on an account but it could be a block on their end.

Hello all,

It is possible that port 25 is closed for new accounts. This is something we have been doing to reduce spam on our network. Please do not think that we are accusing you of sending spam, it is more that we simply cannot accurately determine who is going to send it.

Stopping spam is a constant fight, so we have implemented some restrictions on newer accounts. To qualify for the ability to send out email, we do require accounts to wait 60 days before we can look into removing the restriction. Once your account has reached 60 days of stable use, please do reach back out to us so we can look into this for you.

Hope that helps!
Regards,
Alex