This may be a coincidence but I'm sharing in case it's worth anything: When I had this issue (game "semi-freezing" when sim gets to the mailbox), I'd observed that I always had it on "Ultra Speed through current action" when the problem arises, so I tried having my sim Get The Mail again at Normal Speed, and that time it didn't freeze. I've been doing that from now on and so far I haven't had a problem.