That's the difference between IMAP (mails are on server) and POP3 (mails are on PC).
So, I bought a new PC and I wanted to transfer my emails from my old PC to my new one. I located the files on my old PC and I copied them to my new one. But when I exported them I kept getting the message "your imap server wants to alert you to the following you exceeded your mailbox quota is exceeded,0". I guess this is because my email server is full but I want to store my emails on my PC. Please help.