I'm able to set it up on Linux (CentOS, Debian, Ubuntu, RHEL), FreeBSD and OpenBSD. Mailer software are either Postfix or OpenSMTPD. You need to specify if it would be set up on a VM and which provider (Amazon AWS or somebody else) or on dedicated real server. I would suggest set-up of at least two instances for mail (one for incoming mail, receiving it from the entire world and sending only to the second instance, while the second instance could host mail, allow logging in by users and sending e-mails worldwide). Regards, Branislav