lazyweb: what's your preferred mail server setup docs on Unixoids for someone who's good at admin stuff (and thus doesn't need automagic things, esp if they clobber existing setups) but has no clue how mail works? an org's server needs mail (esp down the road for automated application mails) and I'm not paying for Office 365/Google Apps.
@calvin OpenSMTPD while developed as part of the OpenBSD operating system is portable. Safe yourself a headache and use it :)
In general it boils down to OpenSMTPD + Dovecot + Roundcube for a sane baseline setup.
Greylisting (spamd) cuts spam a ton but proper handling with big senders like Gmail is tricky. On top of that some spamassassin/rspamd and you're set.
Plus/minus dkim/domains setup etc dance.