It means that "hosting" your own email really means hosting your inbox, and only your inbox. Because "the bigs" have basically monopolized sending email, you will tire of working through everyone blocking email from your 2-bit server. You will cave, and eventually use one of those "bigs" to do the sending half of "hosting your own email." Which means they will "see" and index all your email anyway, which WAS THE POINT most of us were TRYING TO PREVENT IN THE FIRST PLACE. I gave up about 8 years ago, and gave everything to Apple.