That user is just a local user used by iRedMail to manage the mail system. Once you’ll be integrated into AD you won’t need or ever use this local postmaster user to maintain accounts database. All your accounts will be managed from Samba Active Directory.

… like postmaster user mention by you at point 16? OR without this user in AD we can finish the ssetup ?

You add user to AD server. The AD database will be queried by the mail server.

So, you say something like this:

1. You can use iRedMail using ldap from an AD (private) like .local and send/receive emails to/from any Internet mail server (without any other services/servers, like a mail relay or whatever)
2. You can use iRedMail using ldap from an AD like .example.com and send/receive emails from any Internet mail server(without any other services/servers, like a mail relay or whatever)

For 2. I agree with you, it is possible, but like I said, if this AD is the only AD in this environment, it is not a very wise idea. Even if you create a separated OU only for persons who will use this mail server(in this case you will lose SingleSignOn advantage).

For 1., yes I see many broken mail servers that were trying to send mail from an address like user@domain.com.local, or others funny things like this.

Have a nice day Matei!

What I am trying to say is the fact that any piece of software could be setup so it can work in many ways, but in some cases, it is not wise/secure to try something like this. You can for example to use a logon http page, it is working, but it is not safe, and sometimes you could have big problems if you use http instead of https.