If you're patched up it's probably the omni-plant gift glitch JSBrantner describes. If you're not, then you might be experiencing the same glitch I had. A dead sim sent a wedding present to the family from beyond the grave, and that broke the mailbox forever!
My sims couldn't recieve anything that came through the mail; wedding presents, certificates, bills, omni-plant orders, written novels, etc. Everytime one of them checked the mailbox they insta-jumped away from it. I tried deleting and replacing the mailbox like you did, I tried moving the family to a different lot to see if it fixed it (didn't work), I tried saving a copy of the family to the bin and then placing them on a new lot (didn't work). Fortunately the family had the No Bills Ever lifetime reward, so there weren't weekly reposessions, but it was pretty frustrating. Never did find a cure for it, unfortunately.