Originally Posted by falko
Please contact your ISP's support to find out if they block port 25.
No, they don't block port 25. All ports are actually opened on the type of internet connection we subscribed. It is the disgression of the subscriber to filter out the unneeded ports by having a firewall (either hardware or software). In my existing network set-up prior to the acquisition of the Anti-virus and Anti-Spam/Firewall hardware I configured my Linux Server as Web/Mail/DNS and Firewall Server. Opening only certain ports for access (HTTP,SMTP,DNS,IMAP,POP).
When I installed the new hardware, I disabled the Firewall at the server.