Deleted all email but the Message Center Storage still says 100% used and people are unable to email me because "quota is full"
The mail in your Trash folder still counts against your quota. Be sure to empty the Trash folder.
If you trash folder has been emptied one other option is to forward some of your email to another service (or program such as MS outlook) & deleted some of your VZ email that was forwarded. If VZ has added a feature to download directly from the web interface I would be very interested in this and would certainly help in this case.
