There are a couple of things to point out.
The company you mention (http://www.gevers.eu/
) has a section on their website stating that a lot of fake e-mails have been sent out pretending to be from them. Looking at the e-mail that has been sent to you, it just seems a bit "off". Maybe its worth someone sending the body of the e-mail to the company with vital details removed to see if it is a standard template that they use.
Firstly, I've never known a company to just send an e-mail out of the blue demanding a very rounded figure of 2000 EUR. Normally it follows a chain of warning e-mails and letters.
That said, seems Studio Peyo went on a rampage in 2005 trying to get websites shutdown with even the slightest reference of the Smurfs, even one that was created as a joke translating English into smurf "http://www.theregister.co.uk/2005/11/08/smurf_domain_revenge/". However, from what I gathered, none of them were sent outright e-mails requesting payments.
Where is the line? That is the question. It's a very grey area. You could argue that your game is a tribute, or a historical reference under the fair use acts. Games TV's shows or magazines use clips from almost every game, do they have to seek permission before they can show a movie of it!?