Nick
Bouncing emails for unknown recipient in the local domain is the default for Exim (and probably most MTA's). It sounds as if your postfix/virtual file might be the answer - you need to direct all mail sent to bounce--* to the mailbox configured in mail-lite.
Exims' equivalent of this - the alias file can't handle wildcards in the same way so I think in Exim I need to define a new transport to direct the email for local delivery. I will have to get my hands dirty in the config to work this out.
Steve