Define your site properly in Domain Name Service (DNS). You must have properly defined PTR and A records for your mail server. The IP address must map to a valid host name using a DNS PTR record. That host name must map to the connecting IP address using a DNS A record. In cases where there are multiple PTR records, ALL returned host names must map back to the connecting IP address. In cases where there are multiple A records, AT LEAST ONE returned IP address must match the connecting IP address.