[sumtechguy]

Another issue that needs to be solved is 'bad actor'. Basically someone gaming the system and overloading it with spam. There is no real nice neat way to fix that. Instead it is a mishmash of blacklists/whitelists/blocklists and sorta intelligent alg filtering. Getting that all filled and working is not trivial either. Oh its 'doable' but kind of a pain for even someone with decent experience at it.

[ywei3410]

To add to that; with re-assigned IP's, widespread port 25 blocking /most/ ISP connections won't allow you to run an email server on a residential connection.

Spam/botnets have a lot to answer for.