There are many different reasons for being added to a black list. The most common one caused by the fact that ORDB.org includes mail from postmaster@yourdomain as part of their relay testing system. Even though this isn't really technically a relay, they blacklist sites that allow mail from that address without first forcing authentication.

In MDaemon you can do the following:

1. Open Accounts | Address Aliases.
2. Enable the option for MAIL FROM Postmaster requires authenticated session.
3. Submit your site for re-testing at http://www.ORDB.org. You will be removed from the blacklist.

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