What does the html source look like? (for the link tag)  And does this happen if you click from a non-episerver site?

Det html looks like this: <a href="mailto:info@company.com" orgrl="mailto:info@company.com">Mail</a> What is the orgurl? It seems to be something the editor puts there. I tried to delete it, but when I publish the page it is back.

I havent seen/heard that this is a problem on any other site. And on this site it only happens on some computers. On some the mailto links works fine. 

I've tried this in a EPi4 site, and it's not possible to remove the orgurl element. If you switch between html and normal layout, it is back in the code...

To confirm this as the error source (or eliminate) you could try to change the html before clicking the mailto link. If a user have a page where he/she always get the error, open the dev toolbar in IE, change the HTML and then click the url. Does this give the same error?

This sounds like a really strange problem... You should contact support (or browse the bug list) to see if they have experienced this before.