Are you sure you don't have an firewall that blocks the port?
I have succefully implemented an mail server (postfix-dovecot) on DO and never noticed that DO blocks ports (would be strange).

Thanks for the reply. Yep, I'm sure. I tried several things including making sure the firewall is open. So I contacted support and they replied with:

*"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.

We appreciate your understanding on this matter."*

I hate it when companies say "We appreciate your understanding" - You're assuming that I'm understanding and I'm not! I'm frustrated! It's a terrible user experience, and surely all it means is that legitimate users and spammers have to wait 60 days.

I understand your frustration.

If SMTP is not mandatory for your web app and all you want is sending emails from DO via Zoho, you can try Zoho mail API.

I don't think DO blocks this. API is usually more performant than SMTP.

Reference:
https://www.zoho.com/mail/help/api/post-send-an-email.html

we have the same problem.
more than 120 days and digital ocean does not unlock our emails
we are spending a lot of time trying to find a solution
but we know it's blocked by digital ocean
lately they are forcing us to look for another company
I'll open a call on the linode, they have good answers.