There are several important settings you can add for your sender domain.
Here are the settings for authenticating your domain:
Spf record:
http://blog.mailrelay.com/en/2012/01/23/spf-protocol-an-ally-in-the-fight-against-spam
Dkim key:
http://blog.mailrelay.com/en/2012/09/27/what-is-the-dkim-system-and-how-does-it-work
Custom domain:
http://blog.mailrelay.com/en/2012/01/18/setting_custom_domain