High load of Read HDD with one mailbox.

Hi, I have ISPconfig3 on Debian6. On the server I have many mailboxes, but one mail box do lot of read of disk.

When client connect than mailbox starts read. That reading is still active even if client turn off PC.

I dont know where I cant start. Can anyone give me advice?

 

And that doing only one client who have notebook Apple Air.