[exim] How to handle DEFERring ACL condition

In acl_check_data, I have a bit like this:

  warn
    condition = ${lookup dnsdb{ptr=$acl_m_data_rip}{false} {true}}
    set acl_m_data_noptr = true

with downstream rules looking at acl_m_data_noptr to decide how to
dispose with the message.

acl_m_data_rip is not the connecting host IP, so I can't just use
sender_host_name and related facilities.

The question is how to handle the case when the lookup times
out/DEFERs. From the POV of the downstream rules, this case is
different from either success or failure - I should really like to say
4xx in the SMTP session. But there seems to be no "third way" out of
this context. Or is there?

--
Please *no* private Cc: on mailing lists and newsgroups
Personal signed mail: please _encrypt_ and sign
Don't clear-text sign: http://cr.yp.to/smtp/8bitmime.html