Mails get not deleted after QUIT

[DarthSonic]

var messageCount = client.GetMessageCount();
for (var messageId = 1; messageId <= messageCount; messageId++)
{
    var pop3Message = client.GetMessage(messageId);
    // do something with message
    client.DeleteMessage(messageId);
}
client.Disconnect()

With this code E-Mails get not deleted after Disconnect. Why that?