Creating a relay like craiglist

[talkingtab]

I am trying to do something very much like craigslist where I give out local addresses (like 1234@keyboxmail.com) then translate those addresses to a real address. So I need to change both the source and destination

To: 1234@keyboxmail.com => To: fred@example.com
From: Jane@example.com => From: 4566@keyboxmail.com

I've been able to build a rcpt_to plugin and I believe I can get the addresses from the transaction. And change them?
Or hook at different point? or ... ?
Just looking for a little guidance on the best way to do this.

I looked at the alias plugin and it appears that
txn.rcpt_to.push(new Address(<${addr}>))
works, but I don't know if there are any gotcha's
Thanks!
Terry