Hi Digi-Quick,
I have such a setup running in two cases.
(with no problems - runs just fine)
The iRedMail server uses fetchmail to receive mails from an external MX (at the provider).
(This only required fetchmail to be installed and configured)
The iRedMail also uses a SMTP client connection back to the MX to send mails.
(This is necessary, as without that such a 'local' mailserver (not listed in the MX record) would be
treated like a zombie PC of a spammer by many recipients (and blocked))
If you still/again consider using iRedMail I can post you some hints for setup
(Or create a mini HOWTO for a local mailserver)
I also use an iRedMail based local mailserver (in a DMZ) to serve a few Android mobilephones via
IMAP IDLE (standard conform push functionality). This also works very fine (using K9 on the android devices)
Best Regards from Vienna,
Marcovaldo