Problem of mail server

I dont have an answer myself at present...just questions.

1. Virtual server location...who is it hosted by
2. You have opened port 25 on firewall (if enabled)
3. What install method was used for the AMP itself and virtualmin.

Personally i found that so long as server firewall was right and i fllowed the virtualmin setup document exactly...it essentially works straight out of the box. I used debian rather than centos...no particular reason other than im just a bit more used to the debian/ubuntu terminology in command shell (im a code tragic though)

The trouble is, unless i know your server ip or your domain url i cant perform any lookup tests to see why its timing out. I dont know what the error you posted means from a programming perspective...just basing my reply on my own problems getting a google cloud mail server running....which is not your provider obviously. I have no idea of any of your server and email configuration. However, Essentially timeout means communication is not getting through. This could be caused at your end or the other end. You cant necessarily do anything about the other end, but you can certainly woek on your own end. Sometimes the other end issue is that your end is not configured correctly and so it rejects your attempts at communicating. On your end...Are you 100% sure you vps ports really are open and that your hosting provider has not network restricted smtp outgoing ports to prevent spam? Google do this...you can recieve but not send. We need to port scan your server to be sure. A couple of options come to mind...

1. Dont try non secure email through port 25...Use TLS on a different port
2. route all outgoing email through a relay service like sendgrid on a non standard port. Sendgrid then change the email to a standard port and forward it onto recipient for you. Sengrid has a free option.

Have you checked documentation at your service provkder..eg for firewall start here .https://documentation.online.net/en/dedicated-server/tutorials/security/...