SCP Agent: What's in the box?

SCP Scientist: Nothing

SCP Agent: So, what's the big deal? Just open it. Or toss it out. I almost tripped on it and spilled my coffee...

SCP Scientist: You don't understand. Our measurements show that there's nothing in the box. It contains, or rather doesn't contain, a complete absence of space, time, and matter. It's a hole in the universe that is inexplicably cordoned off from the air in this room, and everything else in it, by a flimsy cardboard shell.

SCP Agent: ...

SCP Agent: Is that bad?

SCP Scientist: Very.

If it can be sent by mail it is probably not a Keter class object. Object class does not imply danger level

Tho if it randomly appears in mail it maybe is