<p>My Aim: Sending an email (e.g. support request, error report) out of a C# Windows forms application. I know how do do this technically but here is the catch:</p>

<p>In order to authenticate myself on the mail server (e.g. gmx, yahoo, own website) I need to store the login details inside the application (which runs on the client system). If someone extracts it or sniffs the network traffic he will have access to my email account. Maybe the second threat could be eliminated using a secure connection. However, it is still possible to get the password out of the application regardless if it is obfuscated or not.</p>

<p>So is there are 'secure' concept enabling me to send e-mail messages out of my program, which runs on the client pc without the need to store a password in accessible form and not requirering a special web service?</p>

I had such a requirement in the past which we solved by using a web service to post messages.
If for example a user wanted to submit a bug report it would be turned into a web service call to our web server and it would then be forwarded accordingly.
To answer your question:
There is no way you can hide you mail server. If your program can send emails, so can the user with another program. (the same goes for a web service.) What you can do is use an open smtp server (smtp relay) but that is just asking for trouble.