Q: why can't I delete my mail out of my mailbox? When I reopen my mail app later, all the messages I deleted are restored.

I have thousands of emails accumulating in my mac mailbox. I've tried deleting them all over and over again, but if I quit mail or shut down my computer, all of my emails are restored after I reopen mail after. I want to be able to delete my messages and never see them again!

Thanks for your help in advance!

-Peter