ReplyThere was someone who wrote an online interface to be able to send e-mails and have them show up as Google Talk messages which may still work but Google Talk and XMPP have been obsoleted by Google and everything is moving to Google+ hangouts by the end of the year which is proprietary and therefore Google Talk will no longer work. Before you spend to much time setting this up I would verify what your customers plans are once XMPP goes offline later this year or early next.
https://emailtogtalk.appspot.com/
-Jeremy