Galvanization is the process whereby normal steel is electroplated with a high zinc coating on the surface of the steel. This high zinc coating is rust resistant. If you have steel that could be exposed to moisture (like a locking mailbox made of steel) then one of the ways to help it resist rust is to have the steel galvanized.
However, galvanization has several problems when galvanized steel is used to make steel mailboxes. The biggest problem is that wherever the steel is cut, it forms a raw, unprotected edge. Also, if the steel is welded, the galvanization is destroyed at the weld spots. The mailbox is a lot more likely to rust in these places.
Most of the rust that I have seen on steel mailboxes has come from the fact that the metal underneath had the galvanization destroyed due to cutting or rusting.