Sending e-mails with attachments from webplayer

I know that I could open another window with variables passed in the URL and I could point to a file that exists somewhere, but the thing is that I'd like to send a screenshot - therefore it'd need to get generated on the server, but it seems impossible... right?

No it doesnt. in the Unity3D scriptrefences / docs they even have an example of it:
http://unity3d.com/support/documentation/ScriptReference/WWWForm.html

You dont mention if you know it or not, but to be able to send a screenshot from Unity3D client, you need to grab the camera pixels and generate a PNG from it. This PNG you can then transfer to any webserver with WWWform as binaryfiledata. Just like a normal FORM does fileupload.

On the server, you will need serverscript access. Such as PHP, ASP or ASP.NET.

I’ve made an ASP.net page that receives my screenshots without a fuzz. Found it all by using Google.

Please let me know if this still is troubling you.