refused by the DNSBL dnsbl-1.uceprotect.net

refused by the DNSBL dnsbl-1.uceprotect.net

When I try to send e-mail for a client I get following error;

<reciever@recievercustomer> INVALID_ADDRESS, ERROR_CODE :501, ERROR_CODE :5.7.1 <myaddress@myadrees>... IP 136.143.188.55 refused by the DNSBL dnsbl-1.uceprotect.net


I don't get that for every clıent but for some.