My mum played with her Microsoft Lumia smartphone in the evening and she changed some settings of her phone. I do not know what she changed, but the result was that she downloaded 7.500 e-mails. Yes you have read the amount correctly 7.500!
And i do not have this amount of e-mail in my e-mail box. It seems it where already deleted e-mails. So i do not know how my mum was able to download them, and what kind of magic trick she did to make this happen. What I do know is the solution if you have the same problem.
The solution of this problem is to delete all your e-mail accounts in your phone. After that add them again so that you still can receive and send e-mail with your smartphone. My mum's phone works like normal again, because all the settings she has changed are back to the standard settings.