Hi Zhang Huangbin:
I believe your decision to drop support for CENTOS 5 is justified and appropriate.
All operating systems have a prescribed life cycle. For RedHat Enterprise products (and derivatives) this is longer than most Linux distros at 5 1/2 years. Debian for example is typically three years.
The iRedMail community needs to embrace change and recognize that you do not have unlimited resources to support legacy systems. It is not realistic for you to support multiple forks for iRedMail and iRedAdmin Pro to accommodate operating systems which do not support updates to current mainstream mail packages such as policyd.
I think most of us can agree that you do a stellar job of supporting the iRedMail community and we should zealously support this decision as in our best long term interest.