Okay we did some further investigation and everything points to an amavisd problem here. The error came with the update from amavis 2.7.x to 2.8.0.
Referring to this thread here http://www.iredmail.org/forum/topic3809 … sdnew.html we think it's an config issue.
With this it seems to work:
$sql_select_policy = 'SELECT "Y" as local, 1 as id FROM domain WHERE CONCAT("@",domain) IN (%k)';
but we get an other error then
451631A21C5 12293 Thu May 9 11:36:03 firstname.lastname@example.org (host 127.0.0.1[127.0.0.1] said: 451 4.5.0 Error in processing, id=24147-02, spam-wb-list FAILED: sql exec: err=1146, 42S02, DBD::mysql::st execute failed: Table 'vmail.wblist' doesn't exist at (eval 115) line 172. at (eval 118) line 324. (in reply to end of DATA command))