The support team at Digital Ocean removed the SMTP restriction on their firewalls for my account and now emails are sent as expected by ODK Central.
They were filtering packets by protocol, so I could reach the smtp servers at Sendgrid, Mailjet and Mailgun using ping (yes, I have configured accounts on all three 
) but not relay email through them.
If anyone get into this issue I strongly suggest to create a ticket to the Digital Ocean support team right away.
Cheers!