SMTP as post master

Hi folks,

I have been googling around for a Howto/Guide/Tutorial to setup a MTA/SMTP server to route incoming mails to their respective server based on domain. It brought many threads and I hesitate which is the right direction to proceed.

My goal is;

All domains points to the same fixed/external IP.

Several Mail Servers running on a Virtual Box

Guest_1, the SMTP server do the routing (using the DNS with the MX records)

Internet (incoming mails) -> via router forwarding port 25 -> Guest_1 – smtp server -> decides to send mails to Guest_2, Guset_3, Guest_4, Guest_5. etc. based on domain.

Please shed me some light. TIA

B.R.
satimis