@Christoph
Swapping email details is perfectly acceptable as long as it's done away from the public forum.
I'm not happy to share that information (certainly at this time) though. I find that problems emailed are generally such that would expect me to devote inordinate amounts of my time (not always, but generally). Furthermore, they encourage posters to get away without explaining their problem properly, in the belief that it isn't necessary (as I already have their project available). So I'm not a big fan of that, though I may decide to ask for it sometimes when communication becomes too difficult.
Anyway, for this particular problem, I suggest you review a tutorial I set up a while back (
Example Filtering on a Form) as an introduction to such filtering issues. It comes with an example database (very basic) that you can download and experiment with.
Have a play, then let us know here how you got on and if you need further help.