more than a day. still no response. what a poor customer service.

Does this thread help? Specifically the section titled “Are your SMTP connections being blocked?”

• https://meta.discourse.org/t/troubleshooting-email-on-a-new-discourse-install/16326

You could also check to see what ports are listening on your droplet:

$ ss -lntu

The centre column of output is what you’re looking for - port 25 for SMTP.

If you have a firewall like iptables or ufw active, then this could be causing the problem (perhaps).