Basic-Concepts
Web Servers
Uptime
Internet
Programming
Version Control
Configuration Management
Virtualization
Databases
Mail Servers
Domain Name Servers
Some useful links to explain the concepts of SPF & DKIM
http://blog.postmarkapp.com/post/86506131503/explaining-spf
http://blog.hyperfive.com/2011/05/spf-records-explained.html
http://www.openspf.org/SPF_Record_Syntax
https://www.digitalocean.com/community/tutorials/how-to-use-an-spf-record-to-prevent-spoofing-improve-e-mail-reliability
https://cheriches.com/linux/mail-server-series/2014-09-07-dedicated-mail-server-hosting-multiple-domains-step-5-antispam.html
https://kura.io/2015/01/03/debian-wheezy-tls-mailserver-with-mysql-clamav-domainkeys-dkim-spf-solr-imap-search/#ssl
http://www.stevejenkins.com/blog/2010/09/how-to-get-dkim-domainkeys-identified-mail-working-on-centos-5-5-and-postfix-using-opendkim/
https://easyengine.io/tutorials/mail/setup-opendkim/