[exim] per-user transport_filter

If I set recipient limit to one and define transport_filter for smtp
transport, then I can do per-user spam scanning?
If I use queue_only and queue_run_max/remote_max_parallel, I can also
limit number of scan processes?

local_scan_smtp:
driver = smtp
max_rcpt = 1
size_addition = -1
transport_filter = /usr/local/bin/spamc -u ${local_part}@${domain}

Nice thing is, that transport_filter is run only when connection to
remote server is established.