Snails in my Mail... Curses

I don't know how common this is in Melbourne, but almost every time we get a fair amount if rain, I get snails in my mailbox. I've found up to two at a given time.  The odd thing is that there are no holes large enough for the shells to get in. Plus my box is always locked.  I think it's a mailman conspiracy. Do they mature after already being in the box? The funny thing is that the snails will eat my mail. It would be great for children with bad mailed report cards... Not so great when bills are unreadable.