This company The Radicati Group, Inc. produces “quantitative and qualitative research on email, security, instant messaging, social networking, information archiving, regulatory compliance, wireless technologies, web technologies, unified communications, and more.” Looks like one report will run ya about $7,500, but they publish summaries, including the email one. It has some interesting stuff in it:

In 2015, the number of worldwide email users will be nearly 2.6 billion. By the end of 2019, the number of worldwide email users will increase to over 2.9 billion. Over one-third of the worldwide population will be using email by year-end 2019.

Over the next four years, the average number of email accounts per user ratio will grow from an average of 1.7 accounts per user to an average of 1.9 accounts per user

As in, most people have 2 email addresses.

In 2015, the number of business emails sent and received per user per day totals 122 emails per day. This figure continues to show growth and is expected to average 126 messages sent and received per business user by the end of 2019.

They say “consumer email” continues to grow as well, but mostly because of stuff like notifications and not so much “interpersonal communication.”