You can only have a maximum of 500 messages stored - this includes your INBOX and SENT and any additional folders you might have set up.
If you scroll all the way down your PRIVATE MESSAGE page, you'll see a tan FOLDER CONTROLS section.
The next line states:
Inbox contains X messages. You have Y messages stored, of a total 500 allowed.
Note the # of messages stored also includes INBOX and SENT and the other folders.
Until your total stored amount is under 500, you will not be able to send any messages.
Last edited by MTS; 11-11-2011 at 04:17 AM.