Chuck there is a limit of 50 messages in each folder.
If you delete a message it goes into DELETED folder.
Once there's 50 in that folder it won't accept more.
Also if you do delete some so you have 40 messages in a folder you'll see just above written "release held messages".... click on that.
Let us know if that works or not please?
Just checked the folder options in the message area & selected to 'delete the oldest' if the box is full... then tried deleting another form my in box.