I get these for a subaccount. It claims 213MB of 256MB are in use, but when I check the mail by connecting to verizon online (webmail.verizon.com) it shows only 38 messages, none larger than 100K. Additionally, the "message center storage" icon tells me, when I click on it, that less than 1% of the storage is used (consistent with only 38 small messages). Other folders, e.g. trash, are empty, and there is nothing under "My Folders".
Who are you getting these messages from??
I sometimes get several messages a day, now close to 500 spam messages since last year, saying my mailbox is almost full, and my service will soon be shutoff. Checking who they came from, it turned out all to be spam.