I lack the expertise to elaborate much, but the gist is that people running their own server were caught by measures that were put in place to combat spam. I recall some providers (AOL?) used to be infamous for silently discarding your mails, which is very aggravating for the involved parties.

You had to jump through certain hoops to ensure that your mail arrived. DKIM/SPF are two such hoops (or related techs, anyway; both have detailed Wikipedia entries). Maybe it's not so much of an issue these days.