This happens because your allocated space for your email has been used and you don't have more space for other emails. Possible solutions to solve the problem are:

1. Delete unwanted messages from your Inbox. Messages with attachments which indicated by a paperclip symbol beside the sender's name take up more space.

2. In the Sent Items folder delete messages that you no longer need. The Sent Items folder makes a copy of every message that you send.

3. Delete the messages from the Deleted Items folder.