[Exim] hints database problem

I am using callout in a RCPT ACL for recipients on some local machines. I found
that I was getting frozen non-delivery messages for one particular user on one of
those machines because messages where not being rejected by the callout but were
being rejected for the actual attempt to send. I have eventually worked out that
the callout is not happening because the hints database says the address is OK!

How long will the data in the hints database remain valid? Section 42.6 says "Wxim
ignores any records that are more than six hours old", but that it where it is
discussing ETRN. In my case it is well over 6 hours.

Is it possible that when an outgoing RCPT fails the hints record is removed?

Phil.
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Phil Chambers (postmaster@???)
University of Exeter