Can port 25 be used for SSL-enable SMTP server ?

Question

Hi,
Our customer is using port 25 for a SSL-enabled SMTP server without certificate. When our email client tried to connect to it, the following exception thrown:
DEBUG SMTP: exception reading response: javax.net.ssl.SSLException: Unrecognized SSL message, plaintext connection?
Since we don't want to ask our customer to change their port configuration unless absolutely necessary, we did some tests with our own SSL-enabled SMTP server that uses certificate. Here is what I got:
1) with port 25, got the same exception as above;
2) with port 465, worked fine;
3) with any other randomly pick up valid port, worked fine.
This made me wonder if 25 is for non SSL SMTP server ONLY. By the way, I'm using Javamail 1.3.4 and JSDK 1.4.2_02. My question is whether we can configure javamail so that port 25 can be used by SSL-enabled SMTP server?
Your help will be appreciated.

Answer 1

Yes, port 25 is intended for non-SSL servers only, although that doesn't
prevent a client from making a plain text connection and then using the
STARTTLS command to switch the connection to SSL/TLS. JavaMail 1.4
supports that usage.
You can configure JavaMail to use port 25 for SSL connections if you
really want to. JavaMail 1.3.x requires you configure an appropriate
socket factory to get SSL connections; you can configure whatever port
you want for use with that socket factory.