Hi,
As per your update your mailbox size is around 20GB.
Also you are using outlook 2007.
By default, the limit for.pst file in Outlook 2007 and in Outlook 2003 is configured to be 20 GB
http://support.microsoft.com/kb/830336You are getting this error because your .pst is reached the maximum size limit.
You can increase the size of your .pst by editing the registry.
Follow the below article and edit the registry value.
http://support.microsoft.com/kb/832925It will resolve your issue.
Thanks.