&quot;I&#x27;m not sure that&#x27;s true (see: Sparrow). I just don&#x27;t think there is enough demand for people to want to pay for an email client.&quot;<p>Yeah, this is definitely what I saw with Sparrow. For it to become popular it has to be adopted by a substantial number of people. However, I find that only really tech savvy people cared about Sparrow and its streamlined interface. When I showed it to other people, they were reluctant to use anything else because they were afraid to move from their Apple Mail.  I&#x27;m guessing if Sparrow had existed for windows it would&#x27;ve been the same thing with people using Outlook.<p>And then there&#x27;s people who don&#x27;t even use e-mail, who think it&#x27;s outdated and prefer PM messaging as a form of communication.  It&#x27;s easy to see why a lot of these promising mail client apps don&#x27;t really take off the way other apps and services do.<p>Nevertheless, Sparrow will always have a home in my Mac, as long as it keeps working on OS X.