I installed ISPConfig on Debian (dedicated server), domain had SPF record. When I send an email to the users, there are some target servers refuse to talk. The below message copy from postfix queue. E67CA2007E 1215 Fri Sep 28 00:00:02 firstname.lastname@example.org (host hnicpmaa01.vnn.vn[126.96.36.199] refused to talk to me: 521 hnicpmaa01.vnn.vn Service not available - no PTR record for 188.8.131.52). In addition, when i send email to yahoo, almost to bulk mail, some working fine How do I solve this problem?