[Exim] rejected RCPT (not temporary)

Hello,
I'm using exim 4.41 and everything was running well
until a couple of days when I saw in the logs:

2004-08-16 10:14:57 H=(some.host.name) [some.ip] F=<some.address> \
rejected RCPT <some.other.address>

the host connecting is an smtp relay from some isp, the FROM and
RCPT adresses are both local to my mailserver.

I don't have any clue saying this is due to an ACL or a 5XX error.
Even the bouncing mailer-daemon message (F=<>) gets the message
"rejected RCPT".

I use some blacklists, but I should then see an entry in the
log syaing "blackhole...". It it not due to dns, this is not
a temporary rejected RCPT.

any idea is welcome :-)

julien.

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