This is probably because your Network Administration has set a 10GB maximum mailbox size on all users email accounts. I would assume you are connect to email through an Exchange server on your network.

This is usually done to ensure that mailbox sizes on the network do not get too large. Essentially you either need to archive and/or delete emails you do not need to reduce the size of your mailbox or speak with your network administrator. This is not something you can change yourself within your mailbox settings.