Many ISPs are blocking what is called "Port 25" which is the port used to send e-mail. They are doing this to cut down on the amount of spam that is sent from their networks.
All e-mail sent via the Internet is routed through the port 25, the channel used for communication between an e-mail client and an e-mail server. Even though port 25 blocking will probably become an industry standard, however, the filter can create problems for e-mail servers and block legitimate e-mail as well as spam.
Port 25 blocking allows ISPs to block spam sent out through their networks, but it tends to punish the innocent that have a need to send through e-mail servers other than those belonging to their ISP. The ISPs that block port 25 require their SMTP server to be used instead of the remote SMTP server or a SMTP server running on your computer.
All e-mail sent via the Internet is routed through port 25. When an e-mail server that runs on your computer delivers messages, it always uses port 25 to transmit data to remote e-mail servers. Therefore, if your ISP is blocking the port, your messages will not get through. There are two different ways the port 25 is being used by PostCast Server:
PostCast Server uses port 25 to accept incoming connections from e-mail clients. You can freely change that value in both server and client program and everything will continue to work because all TCP/IP connections are directed to your computer. Unless you block connections to your computer, the program will accept messages using any port number you specify (1-65535).
PostCast Server also uses the port 25 for sending. It connects to remote servers and delivers the messages from the Outbox folder. Exactly the same rules apply except that every remote server expects the connection ONLY on port #25. This is the standard port number and while you can change the port number in the program to allow clients to send the messages internally, the remote servers always use port 25. If your ISP blocks remote connections to port 25, you cannot send any messages. PostCast Server will not be able to connect to the remote servers.
When you send messages from a program on your computer, it uses port 25 to transmit data to remote e-mail servers. Even though you are sending mail via smtp.computers-plus.net, your computer connects to the Internet via your ISP's system. Therefore, if your ISP is blocking port 25, your messages will not get through.
Note: If you change the SMTP port and are still unable to send mail from your domain, you may have to send mail using your ISPs mail servers. Contact your service provider for assistance with this.