[exim] Restricting callouts on a per-transport basis?

Hi all,

I am trying to figure out how to limit recipient verify callouts to only
take place over a certain transport. I am having a bit of trouble
working out the logic I need to use for the config I am writing.

Basic setup is an exim box doing virtual domains with a cyrus mailstore
(also configured for virtual domains). In addition, each domain has a
virtual aliases file which provides for addresses which are not local
mailboxes, and can redirect either to a local mailbox or a remote smtp
address.

I thought I can verify the existence of a local cyrus mailstore by doing
an LMTP callout to cyrus to check if it will accept delivery for a
mailbox - if not, then check the virtual aliases files for a redirect.
However I do not want the recipient callout to take place if the
redirect forwards to an external address - just regular recipient
verification, i.e. check if the domain exists and has appropriate DNS
records etc.

Ideally if I could only do callouts over the LMTP transport and not over
the SMTP transport that would work, but there doesn't appear to be
options for this and I can't figure out how else to do this.

Any suggestions would be appreciated :)

Current exim logic (sorry I don't have a full config yet as I am trying
to get my head around the required logic while writing the config!):

Main domain lists:
domainlist local_domains = @ : localhost
domainlist virtual_domains = dsearch;/etc/exim4/virtual
domainlist incoming_domains = +local_domains : +virtual_domains


rcpt acl:

deny
    domains     = +incoming_domains
    message     = Invalid Recipient
    !verify     = recipient/callout=defer_ok,5s

begin routers:

dnslookup:
    driver = dnslookup
    domains = ! +incoming_domains
    transport = remote_smtp
    ignore_target_hosts = 0.0.0.0 : 127.0.0.0/8
    no_more

virtual_cyrus_mailbox:
    driver = accept
    domains = +virtual_domains
    transport = cyrus_lmtp
        retry_use_local_part

virtual_aliases:
    driver = redirect
    domains = +virtual_domains
    data =
${expand:${lookup{$local_part}lsearch*@{/etc/exim4/virtual/$domain}}}
    retry_use_local_part

begin transports:

remote_smtp:
        driver = smtp

cyrus_lmtp:
    driver = smtp
    protocol = lmtp
    hosts = localhost
    allow_localhost

Cheers,
Colin