While I agree spam is increasing, that's not the most interesting thing I see when I look at the Senderbase graph. The thing that is most apparent to me is that the delta between "Total number of emails" and "Number of spam emails" has increased dramatically in the last two months. This delta (the orange shaded area) should represent legitimate non-spam email. The graph would seem to indicate there are ~34.6B legitimate emails in April 2014, compared to ~24.3 billion legitimate emails in April 2013, and ~14 billion legitimate emails in January 2014.

I see two possible explanations for this.
1. Legitimate email really has increased 50% in the last year, and is more than double what it was four months ago.
2. On a percentage basis, Senderbase is not as effective as it used to be.

I would be interested to see reasons why the first point is true, otherwise I'm inclined to believe the second point is more likely.

Sincerely,
A Concerned Ironport Customer