[exim] Altering tempfail messages under certain conditions

Hi,

as DNSSEC is getting more mainstream, especially here in .nl, we also
get more emails that fail because of broken DNSSEC. Or to be more
precise, because our resolvers refuse to answer for broken DNSSEC
domains (SRVFAIL).

Exim reports to the sending MTA 'Temporary local problem - please try
later', though it logs 'sender verify defer for
<info@???>: host lookup did not complete' to disk.

Can i get Exim to return something else if the host lookup does not
complete? Something along the lines of "The DNS of $sender_domain is
probably broken, contact $your_isp"?

Thanks,
-Sander.
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