Some important notes:
*) It's a BAD idea to run mail service with a DYNAMIC IP address. Too many spammers send spam with dynamic IP addresses, so most ISP block emails sent from dynamic IP addresses.
*) As mentioned in iRedMail installation guide:
WARNING: iRedMail is designed to be deployed on a FRESH server system, which means your server does NOT have mail related components installed, e.g. MySQL, OpenLDAP, Postfix, Dovecot, Amavisd, etc (iRedMail will install and configure them for you automatically). Otherwise it may override your existing files/configurations althought it will backup files before modifing, and it may be not working as expected.
So, don't install iRedMail on your existing server, it will mess it up.
Telling me not to install it on an existing system doesn't answer a single question I had. I appreciate you warning me that it could mess my system up, but that still doesn't answer anything I asked.