Your email address has too many or too big emails stored. Your quota is full. All email accounts have standard quota of 1 GB.
You can choose between two options:
1. Increase your quota. Take a look here.
2. Delete email. You can delete emails in your email client or in webmail. If you sort your email by size, it’s easier to see which emails are the biggest (including attachments). You should delete the biggest ones. Please also remember that there are emails in the folders «sent messages» and «trash».
Remember to empty «trash» when you have deleted emails.