Cannot access iRedmail from some users

==== Required information ====
- iRedMail version:      v2.1.1 (LDAP)
- Store mail accounts in which backend (LDAP/MySQL/PGSQL):  LDAP
- Linux/BSD distribution name and version: Ubuntu 12.04
- Related log if you're reporting an issue:
====

Hi ZhangHuangbin,

Its Shaik here.
I have successfully migrated all users to iRedmail server. I have a small problem, some users are not able to connect to the iredmail server but they can ping. when i change the IP Address of their PC they can connect easily. When i return back the same IP again its blocked.

Is their any particular IP is blocked? Is their any way to add unblock ipaddresses without restarting?

regards,
Tharim