A patch that fixes the add domain functionality to admin/create-domain.php
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Which version of postfixadmin are you using? It looks like the latest SVN version has this fixed already. (It uses $sqlBackupmx for the SQL query and $fBackupmx for the template file.)
However, I changed the code to use a slightly different way (database independend, using db_get_boolean()) to set the boolean values.