Members are allowed space for 200 PM messages. When the PM box gets to 200, senders get an error message that your box is full and their message bounces.
It's a good practice to get rid of unimportant PM messages from time to time, so you have room. If you are selling something, your PM box can fill up quickly.
Login, click on "My Stuff" and select "Messages"
To clean up PM messages, look for the checkboxes over to the right edge on your list of PM messages. Check the ones you want to delete and click the delete button at the bottom of the page. Or you can click the "Check All" button and then uncheck those you don't want to delete one page at a time.
You only have a limited amount of room, so you have to get rid of some to make room for new. Kinda like a closet or a garage.