Okay brief answer time. 

Quick recap:

I lost something recently at a friend's house. My friend mailed it back to me. But it is of no use to me now, from this time forward, since he mailed it.

What was it?

My brother said it was a stamp. And while that works, technically, we were looking for something a little more difficult. 

The answer we were looking for is:

My mailbox key.

So, if he mailed me my mailbox key, the mailbox key would be in the mailbox, and I would not be able to get to it. 

Mailbox key. 

Or a stamp. That works too.