This is some interesting info on the email volume generated per day.
VeriSign (the exclusive registry for .com and .net domains) estimates (PDF) that there are about 2.25 billion email queries per day. However, because of caching, email queries represent only a fraction of the number of emails sent. And VeriSign doesn’t track many huge email-producing domains, such as those that end in .edu. So 2.25 billion is far below the actual number of emails sent every day.
According to a CNN article, in 2001, the International Data Corporation (IDC) predicted the number of emails sent each day would exceed 36 billion by 2005. But in 2003, the IDC reported that 31 billion emails were already being sent daily, and they expected the number to double by 2006. They do offer a recent report that may be more accurate — and you’re welcome to spend the thousands to purchase it!
As best as we can figure then, the number of emails sent each day far exceeds 2.25 billion. It may be approaching 62 billion.