==== Required information ====
- iRedMail version (check /etc/iredmail-release): 0.84
- Linux/BSD distribution name and version: debian 8 jessie
- Store mail accounts in which backend (LDAP/MySQL/PGSQL):  mysql
- Web server (Apache or Nginx): nginx
- Manage mail accounts with iRedAdmin-Pro? no
- Related log if you're reporting an issue:
====

Mar  3 07:17:07 mx2 postfix/submission/smtpd[15351]: warning: hostname mx-ll-49.49.238-59.dynamic.3bb.co.th does not resolve to address 49.49.238.59: Name or service not known
Mar  3 07:18:09 mx2 postfix/submission/smtpd[15351]: warning: hostname mx-ll-49.49.238-59.dynamic.3bb.co.th does not resolve to address 49.49.238.59: Name or service not known
Mar  3 07:19:06 mx2 postfix/submission/smtpd[15351]: warning: hostname mx-ll-223.205.245-78.dynamic.3bb.co.th does not resolve to address 223.205.245.78: Name or service not known
Mar  3 07:19:12 mx2 postfix/submission/smtpd[15351]: warning: hostname mx-ll-49.49.238-59.dynamic.3bb.co.th does not resolve to address 49.49.238.59: Name or service not known
Mar  3 07:21:17 mx2 postfix/submission/smtpd[15351]: warning: hostname mx-ll-49.49.238-59.dynamic.3bb.co.th does not resolve to address 49.49.238.59: Name or service not known
Mar  3 07:48:45 mx2 postfix/submission/smtpd[27107]: warning: hostname mx-ll-49.49.238-59.dynamic.3bb.co.th does not resolve to address 49.49.238.59: Name or service not known
Mar  3 07:49:49 mx2 postfix/submission/smtpd[25446]: warning: hostname mx-ll-49.49.238-59.dynamic.3bb.co.th does not resolve to address 49.49.238.59: Name or service not known
Mar  3 07:50:51 mx2 postfix/submission/smtpd[25446]: warning: hostname mx-ll-49.49.238-59.dynamic.3bb.co.th does not resolve to address 49.49.238.59: Name or service not known
Mar  3 07:52:55 mx2 postfix/submission/smtpd[25446]: warning: hostname mx-ll-49.49.238-59.dynamic.3bb.co.th does not resolve to address 49.49.238.59: Name or service not known

I have try

smtp_host_lookup = native

not working.