Siena had an e-mail storage quota to reduce storage space and, occasionally, you may get a message saying your over quota. This can be fixed by deleting messages and then emptying the Deleted Messages folder.
But, sometimes, it doesn't look like you have too many messages. Your inbox is small, and you've already emptied the deleted items.
There's another place to check: the Sent Items folder. Outlook keeps a copy of every message you send. After awhile, this can take up a lot of space. If you delete these messages and then empty the Recycle Bin, you'll be fine.