With regards to the send message feature, I have tried what you suggested but the core module displays an error:
firstname.lastname@example.org - Notification discarded - potential loop.
Hmm. Insidious! Just double check these:
* Postmaster (core module) is a username (no domain) that is valid here
* Local domains (core) includes your domain
* Postmaster's mailbox isn't autoforwarding
If that's all okay, and |it may well be, then I think you've probably found some sort of bug. But I'm thinking the most obvious cause of an error like that is that you've somehow stuffed a domain name (the documentation, I rather suspect, is the blame here) into the Postmaster field.