What does Mandrill's Sender Reputation metric mean?
Sender reputation is basically a measure of how reputable you are as an email sender. Mandrill measures things like bounce rate and spam complaints in order to determine whether customers are happy receiving your email or not.
Sender reputation also determines the volume of email that you can send. If you have a low reputation, you might only be able to send 60 emails per hour, whereas with a higher reputation you can send tens of thousands of emails per hour.