I had a similar issue recently on a 7830. I created a unique email address that I could assign to the device, "xerox@*******.com) and then I added the public DNS settings for Google which are 8.8.8.8 and 8.8.4.4. This was for a small business that didn't have a domain or DNS servers. You also have to populate the domain field with something whether they are on a domain or not. If they are not on a domain it doesn't matter what you put in there. In my case I used att.local.net as that was their ISP.
You can read more about Google's public DNS here: https://developers.google.com/speed/public-dns/docs/using
Edit: Just read in another post where some users are having to put smtp.googlemail.com instead of smtp.gmail.com.
http://forum.support.xerox.com/t5/Copying-Faxing-Scanning/Gmail-issues-7120-7125/td-p/130148
All the best,