If I try to stab you to death, but I fail, is it a crime?

Generally, as long as there is proof you attempted a crime, it’s a crime. However, punishment is often lower than when you succeeded, if only because there are is damage done.

If in the US, they could be charged with mail theft.

what punishment to they receive?

Dunno what they will receive, but they deserve playtime with the family doberman.