It depends.

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.

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.