Java Mail mystery - SMTP blocked?

Question

I have a Java program which uses javax.mail to send an SMTP message. This program works fine on a Linux box, I want to emphasize that beforehand. When I try this same cod

Answer 1

The problem is due to the IPv4/IPv6 preference of Java. Java tries to use IPv6 by default (?) and my ISP does not support IPV6. However, it's enabled on my Windows 7 boxes by default.

If you are experiencing similar behavior, you can verify this by going to "Control Panel/Network and Internet/Network Connections", right-clicking your internet connection, and selecting "Status" from the context menu. The Status dialog will display two lines similar to this:

IPv4 Connectivity:  Internet
IPv6 Connectivity:  No Internet access

This is the root of the problem - Java prefers IPv6, which it cannot use to connect to the internet.

To fix this, do either one (or both) of these things:

1. Disable IPv6 on your Windows 7 box.

2. Start your java program using this command line option:

   -Djava.net.preferIPv4Stack=true
   

Doing either one of these will fix the problem.