How do I send text messages from an iMac OSX El Capitan to an Android phone? The recipient is not registered on iMessage, which leads me to believe he is not on an iMac.  I have received txts from him on my Android and I have his phone number.  Is there a way to do it on OS X without purchasing additional software?
iMessage uses a connected iPhone to send text messages from your Mac to anyone, including Android. Unfortunately you're using an Android phone so you're going to need a third party app.