We have been down that raod already, we are as far as we know not on any blacklists (that we can find) and according to them we are also not on some kind of blacklist from them.

Acording to the sysadmin it is OUR server creating these mesages.

Translating out of dutch:
Subjects: test
Sent: 29-3-2006

the following adresses were unreachable

[email protected] on 29-3-2006 9:28

The organisation you sent this message to reports that the mail account does not exist. Check the account or contact the account holder.

Our message did not reach all recipeints
<mailserver.domain.local #5.1.1 smtp;550 5.1.1 <[email protected]>... No such user>

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We have similar messages for various domains. about 2 or 3 per day.