Mirco,
It is worse than you think. When you dismiss the reminder, it sends a message to all of those people who are listed on the event. So, for example, if you have a meeting that had say 3 people invited, when the reminder pops up and you dismiss it, it sends and email to everyone on the meeting invite that you have somehow changed the meeting. Quite embarrassing to send an update for a meeting that is a year in the past.
I have had that happen with really old meetings that pop up with a reminder for reasons unknown. I had to select the ‘X’ on the reminder so it wouldn’t sent out an email to everyone. I had to stop using Em Client for business because of that. Tried to get it fixed with pro support but no luck.