Hello,
Thank you for your answer. We do not receive any automatic responses at all. They have got our IP numbers but seems to be waiting for us to fill in domain keys.
Right now the problem is urgent, some of our mail servers has been blocked and we have over 700 000 customers. We already participate in AOLs and Hotmails feedback loop program and suspend spammers as soon as we receive a complaint (or see unusual mail activity in the servers). The last days we have not had any spamming accounts so we have no idea what triggered the block.