The BETA of Office 2010 will expire on 31 October. After this date, the suite will turn into reduced functionality mode.
Reduced functionality mode is a state of the Office suite in which you basically can only view existing data or documents but not create new ones. For Outlook this means that you can still read all your mails and export it but creating and sending new messages will not be possible.
If you are still using the BETA, then you can either uninstall it and go back to your previous version of Office, install a trial version of the released version of Office 2010 or purchase it.