Basically the client reached its quota. So there is no problem with your setup
Thanks for the reply by the way.
Maybe I left out some essential information, sorry
The user has no mail in his inbox on the server, none.
"WebSpace MB: 50" doen't mean that you can send 50mb or so on mail each month right?
It means that you have 50mb space on the server.
But in these case the user has no mail on the server at all.
He donloads it to his workstation on his job.