They are related in that all major email service providers (GMail, HotMail, Yahoo!, Comcast, Verizon, etc) scan email '''content''' on the receiving side with some heuristics to determine a 'spam' score in addition to other more 'technical' measures.
== Audience ==
http://emailclientmarketshare.com/ shows that GMail, which only accounted for 4% of market share in early 2013, accounts for 15% at the end of 2015. Know your market. Know the current devices and technologies in use to read email.
== Content ==