16383 - the magic number! No more messages in Outlook
The situation is this: by default Outlook doesn't allow more than 16383 item in a folder. This is especially devastating if it's you Inbox or Deleted Items folder.
If it's your Inbox you might see a problem something like the following:
You do a Send & Receive, and it says you're receiving mail, but nothing comes into your Inbox.
If it's you Deleted Items folder, you'll probably get some sort of error message when you try to delete something.
To fix this, delete some items from the affected folders. I would recommend deleting as opposed to archiving, because archiving could take forever! If it's your Deleted Items you will have to permanently delete some things, so it's no time to get sentimental.
For more info have a look here: