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 email@example.com (host hnicpmaa01.vnn.vn[184.108.40.206] refused to talk to me: 521 hnicpmaa01.vnn.vn Service not available - no PTR record for 220.127.116.11). In addition, when i send email to yahoo, almost to bulk mail, some working fine How do I solve this problem?