Okay, seems like I found a solution.

You can set after how many characters the line break appears in Outlook 2010 by changing the following setting:
File > Options > Mail > Message format > Automatically wrap text at character

However, you can only chose a number between 30 and 132. It is not possible to disable the extra line break in Outlook. (Note that it even can be disabled in a crappy mail client like the one from Opera.)

132 characters for a line still breaks your layout, so this still isn't a solution. The actual solution (or bypass) is editing the windows registry. I found a hint to this solution here, so kudos to that guy, Stan Starinski:
http://us.generation-nt.com/controll...199009261.html
The mentioned article is this one:
http://support.microsoft.com/kb/823921

So you have to use "regedit" and find the following adress:
HKEY_USERS\(many numbers)\ Software\Microsoft\Office\14.0\Common\MailSettings
And you have to edit the following string:
"PlainWrapLen"

In the registry you can type in any number you want, I use "9999", which means that an extra line break is going to get added after 9999 characters. I guess there aren't many people who will write 9999 characters without using a line break and even if you do, to have one after 9999 characters is bearable. So this is the solution.

I would like to mention that I fucking hate Microsoft and Microsoft Outlook for that kind of crap. Have a nice day.