If you have received a message from System Administrator that you email inbox is full, you will need to delete some of your old messages.

There are three steps to deleting email.  Messages are not deleted until you finish the third step.

Click the messages you want to delete (use the Ctrl key to select multiple messages).

Click the "Delete" button.  This will move the message from your inbox to your deleted items folder.

Click the "Empty Deleted Items Folder" button.  The messages are not deleted until you click this button.  Your inbox isn't the only place where messages are stored.  Use the "Up one folder" button to find other folders--especially your Sent Items folder.  If you have sent a lot of messages and attachments, you may need to empty this folder as well as items in your inbox.  On the right of the screen, each item in your inbox lists its file size.  If you don't want to delete all your old mail, look at the file sizes and try to find larger items.  Deleting one item that has a 1.3 MB attachment frees up more space than deleting hundreds of plain emails.