Another great tip, and very useful for general use - thanks.

But because the searching I'm trying to do is in either the inbox of mailbox a, the inbox of mailbox b, or the inbox of mailbox c, this technique isn't quicker than mouse-navigating. I've set the inboxes as favourites, so it's not much of a hassle. I guess I was simply surprised that there was no purpose-built search operator for specifying a mailbox. Thank you for your tips, though - I'll probably use them for other purposes in Outlook.