[exim] No temporary defer on long lasting callout

Hi,

I'm new to Exim4.5 and need a hint for the following problem:

When a sender verify cannot be completed, Exims responses 4xx temporarily
rejected. But I want to reject mails which cannot be verified in time with
a 5xx. It would be the opposite of defer_ok... How can I do that?

Sincerly,
Christoph