OK - Here's a good trick I get working. I'm not sure how it would work
with real virtual users mailboxes, but it's a step up from just aliases.
The idea here is we have an alias file which forwards what it matches.
But everything that doesn't match gets forwarded on to another domain.
virtual_alias:
driver = aliasfile
user = root
expand <---- important!
file_transport = address_file
pipe_transport = address_pipe
domains = lsearch;/etc/vmail/vdomains
file = /etc/vmail/aliases.$domain
search_type = lsearch*
qualify_preserve_domain
Alias File
althea: althaea
peter: pyronwode@???
family: cat ,nagasiva ,althea ,peter
cat: catherine
marc: marc@???
althaea: azyron@???
test: marc
*: $local_part@???
A message going to joe@??? would forward to joe@???