The amount of space, per mailbox, you have to store your messages.

All POP/IMAP e-mail accounts have a maximum send/receive message size of 20MB. If you have to send or receive files larger than 20MB by e-mail, you should upload them to your web site and provide a link to download the attachment.

If your mailbox reachs your mail limits, your mail will start getting rejected back to the sender with a message your mailbox is full.