Possible reason that email service providers and other email sending services might see an increase in new signups and free tier use recently: http://blog.mandrill.com/important-changes-to-mandrill.html Mandrill in essence raised their price, eliminated free tier, and required prepurchased blocks of sending capacity IIRC

The numbers Mandrill released about that business suggest that they had a large number of low volume senders, who may now be looking for a new home.