Has anyone here tried iRedMail? Having some problems

I'm somewhat new to server administration, and I'm trying to set up iRedMail. It's a postfix/dovecot out of the box e-mail server solution, which installs and configures LDAP to use as it's user backend. Check it out: iRedMail.org

Anyway, I'm having some issues with it, which I'm unsure about because of how new I am to Debian.

I've got it installed and configured. It's supposed to be using Courier and Dovecot. However, instead, when I try to send e-mails to certain servers (mainly other domains, not major e-mail carriers), I get a STMP 550 Error saying my user cannot be validated.

I noticed that my server is still trying to use Exim and Courier instead of Dovecot and Postfix. As far as I know, Exim and Courier do not come installed on Debian by default, and I didn't install them.

I'm sure this is a rookie issue, is there anyone here that may be able to shed some light on what I'm doing wrong before I freak out? :(

Thanks in advance,

-Sam