managing port 25 in windows 8.1 home and cable modem

Question

A cable modem has port 25 blocked. Two computers are on the local wifi network managed by the cable modem. One computer is a windows 8.1 machine that sends and receives email using tools like outlook. The other computer is a Linux development server for testing apps that use javamail to send and receive email on a real web server when they are done development.

When I try to use app functionality on the development server that sends email, I am getting errors indicating that the email cannot be sent.

I think the problem is that port 25 is not available to send email because it is a home network and a residential cable account that blocks port 25 by default.

If I ask the cable company to open port 25 on the cable modem, I can assign an app like postfix/dovecot on the development server to manage the port. But how do I protect port 25 on the windows 8.1 machine without disabling outlook?

Answer 1

Almost all residential ISPs block outbound tcp/25 traffic to anything but their own mail servers. This is done because lots of malware would send spam from infected machines. As a result, you almost never send mail direct from your source to the target server like you used to, but instead "relay" mail via your ISP or some other mail service.

Here are a few options for getting around this limitation:

1. Relay mail via your ISP's servers. They may or may not require authentication in order to send to addresses outside their server. You can usually send unauthenticated messages to yourself and other addresses on that server.
2. Relay via a service that uses TLS/SSL and/or authentication... These services are generally on ports like tcp/465 or tcp/587 which will not be blocked by most ISPs. I use Gmail (settings) for this purpose.