So from what I understand you are sending mails from your server with phpmail.
DO won't block your ports unless you have some spams or something worst coming out from your server, and they will probably inform you about it.
MX records should be set from your domain name server (where you point you NS), so setting MX in DO will only work if you have your domain NS pointing to DO.
Did you try something simple like restarting postfix or checking that its active?
sudo systemctl status postfix, sudo systemctl enable postfix, sudo systemctl restart postfix
You can also try to reconfigure postfix: sudo dpkg-reconfigure postfix