You must've been a paid subscriber in the past to have so many pm's .. I think the limit in that case is 1000 messages. You can choose to contribute again, if you wish .. https://forum.lowcarber.org/forumdisplay.php?f=139. It's not necessary though ....

You can delete 25 messages at a time ... On each page of your inbox (and also sent items), put a check in the box at the top of the message list. This will select ALL messages on the page. Then choose Delete from the drop-down menu at the bottom. Time-consuming, yes .. but not so much as trying to delete one at a time

Quickest option .. if there's nothing you want to keep .. I can empty your mailbox for you with a single click . Let me know