Setting the Hostname & Fully Qualified Domain Name (FQDN) on Ubuntu 12.04 or CentOS 6.4 https://github.com/DigitalOcean-User-Projects/Articles-and-Tutorials/blob/master/set_hostname_fqdn_on_ubuntu_centos.md
How to include multiple domains in an spf TXT Record – Server Fault http://serverfault.com/questions/283125/how-to-include-multiple-domains-in-an-spf-txt-record
Questions & Answers | DigitalOcean https://www.digitalocean.com/community/questions/?q=SPF&primary_filter=newest