I want to migrate a web application from a Windows Server 2003 to a Windows Server 2008 R2. All works fine except sending emails from the application.

If I configure the application to use the smtp server on "localhost" it works, but changing it to the "real" host name (e.g. mail.example.org) no mail is sent. The error message says, that the remote server needs a secure connection or smtp authentication. But since it works when using "localhost" instead of the host name I doubt that this is the problem.

Also it's unlikely a problem with the mail server, I also tried it with another one.

So for me it seems like the firewall is blocking the outgoing connection to the mail server. I tried to open port 25, but it still did not work. Maybe I just did it the wrong way.