Why am I getting this error message: Unable to write the email to the mailbox. Make sure the file system allows you write privileges, and you have enough disk s

I am getting this error message, just starting today: Unable to write the email to the mailbox. Make sure the file system allows you write privileges, and you have enough disk space to copy the mailbox. I have tried several solutions (cleaning out my inbox, compacting folders, rebooting, checking through the webmail side), all to no avail, Can anyone help?