I want to remind everyone there is no dire need to delete messages if you think you need to refer back to them. We have plenty of disk space. But some folks never delete their old PM's and they accumulate, sometimes into the thousands. This just wastes resources, which is why we like people to tackle deleting those they don't need anymore. Do 50 a day and you'll be okay. If folks did this regularly, you wouldn't be confronted with a ton of messages all at once to deal with.
We've gotten back an entire megabyte of disk space in just the last 24 hours, which means there were quite a few messages just sucking up space. But you need not sacrifice messages if you think you still need them.