Interesting (but long and technical) articles about #email, from an implementer of a MUA (Mail User Agent, such as Thunderbird).
Part 1 : architecture ▻http://quetzalcoatal.blogspot.gr/2013/09/why-email-is-hard-part-1-architecture.html Part 2 : internationalization ▻http://quetzalcoatal.blogspot.fr/2013/10/why-email-is-hard-part-2.html