Earlier, when thinking about aliases, we discussed the risk of having to come up with some mechanism for sharing key/correspondent databases between MTA's.
There's another reason to discuss that, namely sites that use separate MTA's for sending and receiving and/or have more than one sending/receiving MTA. Large sites probably have lots of both. Smaller sites might have a setup with one host in some DMZ or similar, receiving all mail and then forwarding it to some internal host that's not reachable from the Internet. Outbound mail might take another route.
As long as all the MTA's are operated by a single administrative entity, this might have some solution outside the scope of this work, perhaps using a central SQL server or similar but I'm not sure. It would of course be nice if we could present a simple and robust mechanism for database synchronization.