how can I fix my email?

I cannot get email, this is what I get when my email is checked; I use Outlook Express:
A message was sent to you that was returned to the sender(bounced)
because it would have caused your mailbox quota to be exceeded.

The following is the reason that the message was over quota:

Quota Type: total bytes stored
Quota Available: 0
Total Quota: 25600000
To fix this problem, delete some messages from your mailbox
I have hardly any messages in my mailbox.
How do I rectify this problem? geo