Research by Dr. Thomas Jackson http://homepage.ntlworld.com/cotwj1/top5downloads.html

The Atos Group, in The Guardian: We spend 40% of our workday on internal mail:

http://www.theguardian.com/housing-network/2012/dec/17/ban-staff-email-halton-housing-trust

Researcher Gloria Mark on email-induced stress, in FastCompany:

http://www.fastcompany.com/3036061/the-future-of-work/you-arent-imagining-it-email-is-making-you-more-stressed-out

Lee Mallon from Rarelly Impossible, interviewed by the BBC:

http://www.bbc.com/capital/story/20150324-the-companies-that-banned-email

Business Insider talks about French company Atos, who came off email:

http://www.businessinsider.com/company-bans-email-2011-12