[exim] How to do callout to specific address?

Hello!
I'd like to use exim to callout senders mta and check if given users
exists[1]. In doc I can see callout is only doing for checking if sender
or postmaster exists. So I assume it needs some more configuration that
adding one word to verify=sender/callout line.
I'd like to ask for some advice.

Second quuestion is can I modify behavior of verify=reverse_host_lookup?
I'd like to set acl_m_xxx depending on success or failure of
verification. Problem appers when dns server doesn't respond (e.g. PTR
record for 117.18.231.4). Acl:
warn verify=reverse_host_lookup
throws warning (no problem) but exim responds 4xx to the client (this is
not ok in this case). Is it possible to catch such cases, set acl_m_xxx
to proper value (e.g. "unknown") and pass to next acl?

I'm using exim-4.80.

Marcin


[1] - I'd like to only check if abuse@ and postmaster@ exists.
(rfc-ignorant is going down...)