Amazon SES hotmail and live delivery error

We use amazon SES as relay host, but our customers can not send invoices (we provide an application than generate invoices) to their customers that have an hotmail, live email. Our new users who try to sing up to our platform cannot activate their accounts from that email providers too.

I checked if the ips was blocked on microsoft or listed in spamcop.net but they are not.

Please help.