I ran into this problem today and found this solution (http://simplyian.com/2015/01/07/Hacking-GMail-to-use-custom-domains-for-free/) took me about 10 minutes to setup and works great!!
+1 for postfix. Here are is another excellent tutorials on the subject:
I’ve tried @simonhoye solution and it works excelent. I’d been struggling with Postfix following as many tutorials I could find but none was enough.
Also found the same solution here: http://dangeroustestlab.net/configuring-mailgun-dns-on-digital-ocean/
I'ts really simple, you have to try it!