Forget installing Dovecot, Postfix, blah, blah, blah, just do it.
At least twice I read about how to not setup an email server using iRedMail because you need to learn the innards of your server. Given the intracacies of email and all that can go wrong, it makes a certain amount of sense. Normally, I would agree with that advice.
Here’s the problem: I tried four different sets of instructions claiming to get you up and running with Postfix, Dovecot and SpamAssassin, and not a single one worked! I mean, these were detailed by the step instructions, yet I constantly hit a roadblock somewhere where I could not attach to the server at all from outside (if I got that far, which one set of instructions did not).
Just follow “How To Install iRedMail On Ubuntu 12.04 x64“. I followed it, and there really is only one problem on it in that he has you define the domain in mydestination and the virtual domains both. One comment on Howtoforge‘s article “Error: Postfix – do not list domain example.com in BOTH mydestination and virtual_mailbox_domains” states that making mydestination to be “localhost” only fixed that error.