[dredmorbius]

Setting up an email server is mad simple.

Setting up an email server that can send and receive mail from other email servers reliably ... not so much.

Most of that is a result of spam and viruses, and the various ad hoc methods which have been institutionalized to deal with both. More often than I'd care to admit, it's the collision between two sites countermeasures and defenses: PGP-signed mail rejected for having attachments, VERP mail rejected for failing to provide a single consistent envelope sender (who but who even looks at envelope sender?), your random residential DSL / server-farm server winding up on a DNSBL, Yahoo pretty much insisting on properly-configured DKIM to receive mail (hey, it's their standard), dictionary spam attacks on your server, remote sites which scan for viruses after accepting your mail (and then blame you for when they don't receive messages).

It's a constant game of whack-a-mole. For basic communications, if you get yourself hosted in valid IP space, it's possible, but if you want high-volume reliable comms, it's getting pretty tough.