Originally Posted by bluegrass
I don't know, but I checked my firewall settings, the same settings was on the system, I have not done any adjustments on it, from the time I install the mail system. When I made an nmap from another server, it showed that port 25 is open.
You need to check outbound. That is, can your server get out to the internet on port 25.
Try this from your mailserver: telnet mail20.ixwebhosting.com 25.
You should see their server respond. If the connection fails, have a look at your firewall again, but look at connections from inside to outside.
Edit: I just realised - it could be your ISP blocking port 25. Many of them do.
On the URLs, yes, I can actually connect to them. But my concern is that why is it that my server seems to send so many emails to different addresses in just a matter of seconds. Is there a freeware tool to check if the system has indeed some sort of a malware?
You could try rkhunter to start with. It's in the Debian package system. It's possible that you've created an open relay which is related to your postfix config rather than malware.